string theory seeks to unify quantum mechanics with general relativity in order to explain everything in one theory... long story short, its got a lot of nasty math and abstract ideas, like... 26 dimensions and all sorts of weird stuff about space-time.
im working on my honours physics degree and string-theory isnt even talked about at an undergraduate level, but if you want a basic non-technical explanation of it, this website
http://superstringtheory.com/ isn't too bad.