http://en.wikipedia.org/wiki/String_theory
There's a good documentary from the PBS series NOVA about String Theory on YouTube - makes it understandable for the layman.
A theory but some of the greatest minds in the world think it's the right path - and if you go back in history, the greatest minds of the day usually were onto something even when they were wrong their 'wrong thinking' usually ended up opening a door to a new discovery made by others later.