If you recall Einstein demonstrated that matter and energy are at their core the same thing, two sides of a coin. (E=MC2) String theory takes this concept to the quantum level.
So at the subatomic level, when you look at the various particles, and work your way down to the most basic building block of these subatomic particles you will get down to vibrating strings and membranes of energy, the manner in which these objects vibrate determine the nature of the subatomic particles. Essentially string theory is the THEORY OF EVERYTHING, an effort to unit the four basic forces of the Universe into a unified concept.
The links include some videos that should help. I think Michio Kaku, the third link might prove the most helpful. His videos are meant for the layman and this one is in four parts on Youtube.
Physicists recognize four fundamental forces in the universe: The weak and strong nuclear forces, gravity, and electromagnetism. Each force is produced by a particle that acts as a carrier of that force - photons carry electromagnetism, while gravitrons carry gravity. The strong nuclear force is associated with gluons, and the weak force with three particles called W+, W-, and Z.
String Theory unifies these particles by positing that these particles are not different from one another, but are actually variations of the same thing, a string. In string theory, the strings (which are actually usually visualized as loops) are in constant motion, and the axis around which that motion is oriented in space determines the behavior, and by extension the identity, of the particle.