I came across this brilliant video of Brian Greene explaining in very simple terms what String Theory is all about. It's a 19 minute video, so make yourself comfy. Brian Greene is one of the most famous string theorists around and he has a knack of explaining things so they are fairly easy to understand. He is also author of the famous book "The Elegant Universe" (amongst others) and the documentary series of the same name.
As many people don't fully comprehend what String Theory is all about, I have posted this link here instead of the science forum, thereby being available to a wider audience. (Mods I hope you don't mind). Enjoy, I am sure you will have a better understanding after watching!
http://www.youtube.com/watch?v=YtdE662eY_M