Possible Duplicate:
Laws of Atomic Theory - how is this possible?
Generally in science, a theory is effectively a fact. The theory of evolution is not a guess, or a hypothesis. It's currently understood to be a fact, backed up by evidence from numerous scientific fields spanning many decades.
String theory on the other hand seems much more contentious, much less settled in the scientific community. Why isn't it called the string hypothesis?
These terms are not used consistently, nor in a way consistent with the way people who talk about science interpret them. For example, the thing called the "standard model" is not really a model anymore (except for the Higgs sector), but an excellent theory, perhaps even a fact of nature, but it is still called the "standard model", not the "standard fact".
An open-ended program you can publish new fundamental papers about is always called a "theory". A "model" is something that was perfectly and precisely well defined in the original paper, like the Weinberg-Salam model. A "law" is a simple mathematical relation that comes either from experiment or theory. A "hypothesis" is a tentative guess, and turns into a theory when you can start writing papers about it. A "principle" is a hypothesis that you really believe in.
These terms are more publishing terms than philosophical terms, and don't give them too much respect.
