sethboy66's response is very good, but I'll summarize it here in case it helps. In science, the word "theory" basically means "explanation." Some explanations are proven wrong, but others have a lot of evidence going for them. Quantum mechanics is a theory that's basically proven, and it's commonly used all the time, such as in the computer you used to write that comment. String theory is not really proven, but it's basically an extension to the existing body of quantum mechanics.