Superstring theory combines relativity and quantum theory in an elegant, intuitive way. First, it describes the myriad of quantum particles of nature because each particle represents a "note" on a vibrating string. Think of a violin string. No one says that A or B is more fundamental than C. What is fundamental is the string itself.

Superstring theory says that, if we had a supermicroscope and could peer at an electron, we would see a string vibrating in a certain mode. The string is extremely small (10-33 cm) so that the electron looks like a point particle to us. If we shake the string, so it vibrates in a different mode, then the electron can turn into something else, such as a quark, the fundamental constituent of protons and neutrons. Shake it again, and the string could vibrate in the mode which describes photons (the quanta of light). Shake it again and it turns into a graviton (the quanta of gravity).

In fact, the collective set of vibrations corresponds to the entire spectrum of known particles. Instead of postulating millions of different particles, one only has to postulate a single object, the superstring. The sub-atomic particles are notes on the superstring. Our bodies are symphonies of strings, and the laws of physics are the laws of harmony of the superstring.

Superstring theory can also explain gravity. When the superstring moves in space and time, splitting and rejoining into other strings, it forces the space-time surrounding it to curl up, just as Einstein's equations predict. In other words, even if Einstein had never dreamed up general relativity, we might have discovered it through superstring theory.

Superstring theory predicts that the universe should exist in ten dimensions, not the familiar four of space-time. To explain where the other six dimensions went, physicists believe that at the instant of the Big Bang, for reasons we don't understand, six of the ten dimensions "curled up" and collapsed, while the other four dimensions expanded rapidly. In some sense, our universe expanded at the expense of a twin universe which collapsed down to microscopic size.

from The Theory of Everything by Michio Kaku.