What Are Strings?
The bassic idea behind String Theory is that all of the different "fundamental particles" of the Standard Model are really just different versions of one basic object:  a vibrating oscillating string. Ordinarily an electron is pictured as a point with no internal structure. A point cannot do anything but move. But, if string theory is correct, then under an extremely "powerful microscope" (way beyond today's capabilities) we would realize that the electron is not really a point, but a tiny loop of vibrating string (sometimes called a filament). A string can do some things besides move  it can oscillate in different ways. If it oscillates one way, then from a distance we see an electron and we are unable to tell it is really a string. But if it oscillates some other way, we call it a photon, or a quark, and so forth. If String Theory is correct, the entire universe is made of oscillating strings. See the illustration at the left showing some strings that might make up a quark.
Perhaps the most remarkable thing about String Theory is that such a simple idea has such a lot of merit. String Theory encompasses Quantum Mechanics, The Standard Model (which has been verified experimentally with incredible precision) and Einstein's General Relativity (gravity). The strength of String Theory is that it accounts for all four known forces in one elegant theory. But it should also be noted that to date there is no experimental evidence that string theory is the "basic description of nature". String Theory is still under development. Bits and pieces of it are known, but we do not yet see the whole picture. We are therefore unable to make definitive predictions about nature. In recent years many new developments have taken place radically improving our understanding of what the theory can some day be.
The most striking feature of String Theory is how small the vibrating string elements are compared to anything in our every day lives. There is no hope that physicists in the foreseeable future could ever measure the size of a string. The electron is two picometers, 2 x 10^12 meters, in diameter. The Up Quark is one picometer. The LHC experiments can measure particles down to 10^19 meters (a millionth of a billionth of a strand of hair). A string element is estimated to be 10^35 meters in size. That is 10 million billion times smaller than what the LHC can measure. Therefore, if we are ever to prove that strings really do exist they will have to be "inferred" from other data. See artist BT Wedge's conception of strings at the left.
Many physicists when explaining particle strings compare them to musical strings, as in a violin or other string instruments. In music, different sounds can be produced depending upon where a string is plunked. Different vibration patterns from a violin string each produce a unique sound. Similarly, in String Theory different vibrations from the infinitely small "rubber bands" result in particles with different characteristics. The strings vibrate at resonant frequencies. Every type of string has a unique resonance or "harmonic". Some harmonics determine a fundamental particle (as opposed to a force) whose mass and other characteristics are defined by the string's resonant pattern. This is in stark contrast to the Standard Model whose many particles have characteristics derived mainly from experimental data. String Theory encourages us to think of the vibrating strings not only as dictating the properties of the particles, but as "being" the particles. The same string harmonics idea applies to the four forces of nature as well as the particles. Different harmonics determine different particles and forces so that all matter and forces are united in one elementary theory. The characteristic patterns of vibrating, oscillating strings provide the music of science. Top
Multi Dimensions
The first versions of string theory, in the late 1960's and early 1970's, had four dimensions  left/right, front/back, up/down and time, just like Einstein's equations and was an attempt to describe the strong nuclear force. However, the mathematics contained what physicists call "quantum anomalies". These are mathematical terms that when interpreted mean the spontaneous creation or destruction of energy, which of course is unacceptable in the real world. In the middle 1970's many physicists abandoned String Theory because of these mathematical difficulties and secondly because only bosons (force particles, not mass particles) were explained by the theory.
Between 1974 and 1984 Joel Scherk of ENS Paris, John Swartz of CalTech, and later Michael Green, of Queen Mary College, discovered that String Theory could encompass gravity in addition to the strong nuclear force. This was a major breakthrough in physics because the mathematics of the Standard Model and Quantum Mechanics could not be enhanced to include gravity in any meaningful way. However, because of the numerous previous problems with String Theory, Swartz, Scherk and Green were largely ignored. However, they doggedly continued their String Theory research (unfortunately Scherk passed away in 1980). In 1984, Green and Schultz presented a breakthrough paper showing that String Theory encompassed all four forces and all matter particles as well. They also presented mathematical solutions to the quantum mechanics conflicts which involved cancelling out the unwanted anomalies. However, there was a new aspect of the theory, String Theory now had 10 dimensions. The presentation ignited a String Theory revolution whereupon hundreds of physicists began working on the theory. This is referred to as the "First Superstring Revolution". Top
Superstring Theory
In the 10 dimensions of Superstring Theory, 9 are of space and one of time. Of the 9 dimensions of space, 6 are "compacted", i.e. the 6 dimensions are curled up into a very tiny space surrounding each point of the normal 3 dimensional space. If the 6 dimensions are extremely small, so that only 3 dimensions can be "seen", there is no conflict with observations. See the active 6 dimensional illustration at the left. It turns out that after the First Revolution, 5 different versions of Superstring Theory were developed, all very legitimate, but of course none could be experimentally verified. Physicists were either trying to decide which one was the best or criticizing the theory saying that all 5 could not possibly be be true in real life, therefore this field of study should be ignored.
In 1995 Ed Whitten, of the Institute for Advanced Study in a paper at a USC String Theory Conference, suggested that there should be 10 dimensions of space plus time, making 11 dimensions in all. If one subscribes to the 11 dimension theory, then it can be shown that the 5 previous versions with 10 dimensions are all special cases of the 11 dimension model and all are legitimate. This paper caused the Second Revolution in Superstring Theory from 1994 to 1997, which was labeled "M" Theory by Whitten (only Whitten knows what M really stands for, possibly Membranes). In the mid 1990s, Joseph Polchinski discovered that String Theory also requires the inclusion of higher dimensional objects, called membranes (branes for short). These added a rich mathematical structure to the theory, and opened up the possibility for including big bang cosmological models into String Theory.
Open And Closed Loop Strings
Joseph Polchinski also discovered that open strings with "loose ends" can not move with complete freedom as one end is attached to a brane. This type of "sticky" branes are called Dbranes. His calculations showed that the Dbranes had exactly the right properties to hold the open string end points very tightly. The "force carrier particles" of the weak, the strong, and the electromagnetic forces are strings with end points that confine them to their Dbranes. Some physicists think that the immobility of these force carrier strings (that are bound to their Dbranes) is the reason these force carrier particles have never evidenced any extra dimensions. However, strings with "closed loops" are "free" to move about. Of the four force carrier particles, the graviton is unique because strings with closed loops are completely free to move from one membrane to another membrane. Top
Supersymmetry  Quantum Mechanics
In the early 1970's Standard Model theoretical physicists were attempting to come up with a Grand Unifying Theory (GUT) uniting gravity with the other fundamental forces. The quantum mechanics mathematics, to be symmetric with respect to particle spin, required one more symmetry, now called "Supersymmetry" (SUSY for short). Supersymmetry theory suggests that every fundamental matter particle should have a massive "shadow" force carrier particle, and every force carrier particle should have a massive "shadow" matter particle. For example, for every type of quark there might be a "superpartner" called a "squark." For every electron there might be a "selectron". See the chart at the left. Keep in mind that this form of Supersymmetry is derived purely from quantum mechanics.
These shadow particles are forecast to be very heavy and therefore hard to observe. Hence, no supersymmetric particle has ever been detected. In the meantime, the early GUT theory predicted that protons would decay. However, after many experiments, there was no evidence of any proton decay and the original GUT theory was abandoned. Nevertheless, many physicists believe that Supersymmetry is real and experiments are underway at the LHC to specifically detect symmetric superpartner particles.
Is there any evidence that Supersymmetry exists? Yes, there is some indirect evidence. In the 1970's physicists discovered that the strengths of three of the four forces of nature (the strong, weak, and electromagnetic forces) converged when extrapolated to extremely high energies. In the 1990's, these calculations were redone with the starting points known with much more accuracy. The results were that the forces "almost" merge, but not quite. This is shown by the red lines in the first chart to the left. (S = Strong Force, W = Weak Force, EM = Electromagnetic Force.) However, if the superpartners from Supersymmetry are added to the equations, the forces "do" actually merge at approximately 10^15 GeV as shown by the blue lines in the second chart. (For an energy reference, the LHC is now operating at approximately 10^4 GeV.) Scientists find it hard to believe that "nature" would "almost" merge these forces. Most scientists believe that the three forces do merge and that this is in fact evidence that Supersymmetry must be "real". Although no precise predictions can be made, evidence suggests that superparticles should weigh from about 200 GeV to 2,000 GeV, heavier than the Top Quark at 171 GeV which is the heaviest Standard Model particle. Top
Supersymmetry  String Theory
The original version of string theory is now called "bosonic string theory" because it only explained the four forces (bosons) and did not include matter particles (fermions). However, the fact that bosonic theory did include gravity as one of the four forces was huge as the main particle theory, quantum mechanics, at that time and even today excludes gravity. In the late 1970's and early 1980's a new version of string theory took hold which did include fermions (matter particles). This was the "Superstring" version which sparked the First Superstring Revolution. This version of string theory included symmetrical particle pairs, now called Supersymmetry, as a fundamental part of the theory.
Most scientific historians agree that the original idea of Supersymmetry originated in string theory, but was also applicable to point particles (zerobranes) as well as strings. Quantum mechanics physicists (who were the rage at this time) incorporated the supersymmetry ideas into quantum mathematics and the theory became known as "Supersymmetric Quantum Field Theory". Most physicists, from both quantum mechanics and string theory, now embrace supersymmetry even though it has not been observed. The original Standard Model had several short comings that Supersymmetric String Theory conveniently addressed. The main issues were incorporating gravity and providing an answer to the "Hierarchial Problem". Superstring Theory, now upgraded to MTheory, is the only theory that encompasses gravity as well as the Standard Model particles and has no glaring mathematical issues. Top
Branes and Mutiverses (MTheory)
As noted above, MTheory is a theoretical extension of the basic string theory in which there are 11 dimensions rather than the 10 dimensions of the Superstring version. The 11 dimensional MTheory unites all previous five string theories and supersedes them. The original String Theory was a theory of "tiny strings" now called 1branes. MTheory incorporated branes into the picture. A membrane, or brane, or Pbrane is a multidimensional medium where the variable P refers to the number of spatial dimensions of the brane. For example, a zerobrane is a zerodimensional point like particle, a 1brane is a string that can either be open or closed, a 2brane is a 2 dimensional "membrane" like a large tablecloth, etc.
Mtheory suggests that objects with large amounts of energy can evolve into branes up to the size of the universe. The value of "P" can range from zero to nine, giving branes dimensions from zero to nine. The inclusion of Pbranes in string theory does not render previous work wrong because of not taking them into account.
Pbranes are much "heavier" than strings, and in the research of extremely tiny "light" particles they can be ignored (as was done unknowingly in the 1970's). Previous versions of Superstring Theory used approximations to simplify the difficult mathematics, but the approximations hid the fact there could be large objects in addition to the tiny strings. Ed Whitten and Joseph Polchinski corrected these early errors and initiated the Second Revolution of string theory, i.e. MTheory.
A strings length is controlled by its energy. The energies of electrons, quarks, and other fundamental particles are so small that the strings that make them up are also very tiny. But if a string had enough energy, it could be quite large. A large 1brane might look like an infinitely long telephone wire. A large 2brane might look like a humongous table cloth stretching indefinitely in two directions. A 3brane could be an incredibly large box. We tend to think of a large box that does not include us or the rest of the universe. But what if the box was so big that the whole universe was inside the box. We would be wandering around in the 3brane just like fish wander around in the ocean. This is called the "braneworld scenario".
The Multiverse. What if there were other "boxes" nearby to our box? Lisa Randall, Harvard Physicist, has suggested that one of the reasons that gravity may be so weak in our world is that it is primarily located in another close brane and "leaks" over into our world.
Welcome to the "Multiverse" (Multiuniverse) scenario. See the artist conception to the left. The basic idea is that in the beginning there really was no one "big bang". There was just "eternal inflation" going on, setting off an endless series of bangs that created many individual universes. Our universe is just one of these many universes.
Elements of our world might interact very weakly, or not at all with other worlds (with the possible exception of gravity). Each world (pocket universe) would develop separately from each other and might even have different laws of physics. It turns out that almost all mathematical models of "early inflation" allow for multiple "pocket universes" to develop. One has to severely restrict the models in order to obtain just a single universe. However, as attractive as these ideas might be mathematically, it is just speculation in the real world. And... there is probably no way to ever test these multiverse theories. Top
String Theory Issues
String theory is an elegant theory that has expanded the thinking of physicists in a number of different ways (supersymmetry, multiple dimensions, big bang origin, etc.). It has also brought very sophisticated mathematics to the physics world and made many physicists into very respected mathematicians. It might ultimately be "the theory" that correctly explains the mysteries of nature. Unfortunately, there is a major gulf between the supermicro world of strings plus the macro world of branes and any observations that can confirm the theory. The theory also has a number of major issues, as listed here:

If string theory is correct, why is it so difficult for humans to detect any dimensions beyond four (rightleft, forwardbackward, updown, and time). The idea of 11 dimensions and the possibility of "branes" are extremely remote from the real world we live in.

String Theory includes far more fundamental particles than have been detected. And those string theory particles that have been confirmed are close to the real McCoy, but not quite exact.

New particles that are projected to exist are too heavy to be observed in the near term foreseeable future. Thus, it is very difficult to determine if string theory is correct or not.

String theory is overly complex. Its future is hindered by its incredible mathematical complexities.

String theory is also very abstract. Its ability to connect with observable data is remote. It seems like a big stretch that some of its implications will ever match the real world.
In summary, string theory implies more particles, more forces, and more dimensions than we can relate to in our world. There are areas of string theory that are within our accessible range. However, we don't understand string theory well enough at the levels of energy we do understand in order to make observations. String Theory is an interesting enigma to physics.