Connections Through Time, Issue 6: January - March 2000
Superstring theory is often referred to as the Theory of Everything. (Most physicists prefer to use the term 'Unified Field Theory' to not be so presumptuous.) 
The superstring theory has as its goal the unification of General Relativity and Quantum Mechanics. It is nothing less than a theory that would explain why the physical-measurable universe is the way it is!
In superstring theory, vibrating one-dimensional "strings", not point-like particles, are the fundamental constituents. The different resonances of the strings create the fundamental particles that we see. Each string is unimaginably small, about 100 billion billion times smaller than a proton, and these strings vibrate in 10 dimensions: 4 space-time dimensions plus 6 still-mysterious dimensions.
So, these vibrating strings lead to the sub-atomic particles, photons, and forces that comprise our universe. Here is a quote from a Washington Post Article, "A Short Course in String Theory (With No Equations)":
"Strings can sing," says Jim Gates, a physicist who specializes in string theory at the University of Maryland. "So you can think of the world around us as a symphony of strings vibrating in different frequencies."
A proton can be thought of as three vibrating strings, one for each quark. Together, their many vibrations play a chord analogous to a musical chord--a combination of notes played together to make a new sound. Instead of producing music, the chord of subatomic strings shows up in scientific instruments as having all the standard properties of a proton--a positive charge, a certain mass and a certain value of a third property called spin. Likewise with the other particles.
For example, the photons that we often think of as particles of light carry the electromagnetic force. And just as vibrating guitar or violin strings produce various tones, so the differing vibrations of subatomic strings--including overtones like those in music--show up in scientific instruments as protons, neutrons, electrons, photons and so on. Much as the sound of the note ąC' is what we perceive from a guitar string vibrating in a certain way, so the attributes that we call "proton" are what we perceive from a string, or superstring, vibrating in a certain combination of ways.
The mass, energy, and forces we observe in our daily lives had their origin with the "Big Bang". Here is the superstring theory interpretation from A Theory of Everything? by Prof. Michio Kaku:
One of the nagging problems of Einstein's old theory of gravity was that it did not explain the origin of the Big Bang. It did not give us a clue as to what happened before the Big Bang.
The 10 dimensional superstring theory, however, gives us a compelling explanation of the origin of the Big Bang. According to the superstring theory, the universe originally started as a perfect 10 dimensional universe with nothing in it.
However, this 10 dimensional universe was not stable. The original 10 dimensional space-time finally "cracked" into two pieces, a four and a six dimensional universe. The universe made the "quantum leap" to another universe in which six of the 10 dimensions curled up into a tiny ball, allowing the remaining four dimensional universe to inflate at enormous rates.
The four dimensional universe (our world) expanded rapidly, eventually creating the Big Bang, while the six dimensional universe wrapped itself into a ball and collapsed down to infinitesimal size.
This explains the origin of the Big Bang, which is now viewed as a rather minor aftershock of a more cataclysmic collapse: the breaking of a 10 dimensional universe into a four and six dimensional universe.
In principle, it also explains why we cannot measure the six dimensional universe, because it has shrunk down to a size smaller than an atom. Thus, no earth-bound experiment can measure the six dimensional universe.
P.S. There is an 11 dimensional theory called M-Theory (where the M is for membrane) that "encompasses" the superstring theory. This is described in a Scientific American Article, here is one paragraph:
Edward Witten, along with Petr Horava of Princeton University, showed how to shrink the extra dimension of M-theory into a segment of a line. The resulting picture has two 10-dimensional universes (each at an end of the line) connected by a space-time of 11 dimensions. Particles--and strings--exist only in the parallel universes at the ends, which can communicate with each other only via gravity. (One can speculate that all visible matter in our universe lies on one wall, whereas the "dark matter," believed to account for the invisible mass in the universe, resides in a parallel universe on the other wall.)
Einstein worked for 30 years and failed to formulate a theory to unify his Theory of General Relativity with Quantum Mechanics. Superstring theory may succeed, and has come a long way toward resolving what were two very different views of the universe. The theory is far from "proven", but it is providing new insights into our universe. Who knows what information may be contained in the additional 6 or 7 "dimensions". Please see the intuition section for some possibilities.
Washington Post Article, "A Short Course in String Theory (With No Equations)"
"The Second Superstring Revolution", by Prof. John H. Schwarz, California Institute of Technology
The Theory Formerly Known as Strings, Scientific American article, February 1998
The Official String Theory Website.
Go to another section of this issue: Consciousness and the "Theory of Everything" Applications: Precognition and Society