There is the fact of evolution - the undeniable* accumulation of data and observations that lead inescapably* the conclusion that all life we have encountered shared, and evolved from, a common ancestor. (*Undeniable and inescapable without some serious mental gymnastics, buckets of untruths (either through forgivable ignorance or less forgivable deceit) and, most importantly, an agenda to deny this scientific truth.) Then, there is evolutionary theory - a body of different theories put forward to explain different aspects of these observations and data. Best of all, in my humble opinion, there is the application of evolutionary theory - my own field of research - in which we use the fact of evolution, the theories about the underlying mechanisms, and the enormous quantity of data we have accumulated to make predictions about areas of biology for which we have less data. Evolutionary theory is used to design lab experiments, manage conservation projects and find cures (and causes) for diseases.
If you are confused about the whole evolution fact/theory thing, it is explained really well in this recent blog post by Larry Moran. In it, he points out that "The Theory of Evolution" (singular) does not really exist. There is no single theory that encompasses all aspects of evolution. (Even the current observation that all life on Earth evolved from a single common ancestor is not "The Theory of Evolution" - evolution would be just as true if it turns out there are multiple unique ancestors, although it would take some serious new data to suggest this.) So, he argues, let's stop using the phrase, which gives Creationists something to distort and confuse people with. I agree.
It's late, so my own summary will be more succinct... Evolution. It works, bitches!