It has become quite popular among Creationists to claim that evolution is "just a theory" and therefore not a proven fact. This was echoed by Ronald Reagan in a campaign speech given in Dallas, "Well, it is a theory [referring to evolution]. It is a scientific theory only, and it has in recent years been challenged in the world of science--that is, not believed in the scientific community to be as infallible as it once was." This view arises from the misunderstanding of what evolution is and how the terms fact and theory are used in science. Before getting to the evolution part, it is necessary to take a step back and find out what is meant by scientists when they use the terms "fact" and "theory".
In science, the word "fact" is used to indicate that an idea has been confirmed to such a degree that to deny it would be crazy.
Furthermore, a fact in science does not mean total certainty. Only in mathematics can something ever be completely certain. The word theory on the other hand, differs much from its use in vernacular language. In everyday speech it is used to indicate simply an educated guess, something on a lower level than a fact. When theory is used in connection with science however, it means something different. Theories are explanations employed to explain and interpret the facts. In the case of evolution, the factual part is our shared ancestry. That we share a common ancestor with other primates is not contested. What is not certain however, is the how. Whether human evolution is mostly the result of natural selection, genetic drift, or some yet to be discovered mechanism is the theoretical part of evolution. Thus, in a way, theories are actually on a higher level than facts.