To expand on Dr A's excellent post on fact and theory:
I explain it to new science students that a fact is an observation. In the example of gravity, the observation is things fall. Why do they fall, how do they fall, that is the theory.
That things fall = observation = fact
How things fall = Theory of Gravity = (G*(M1*M2))/d^2 Where G is a gravitational constant, M1 and M2 are the masses of the bodies in question, and d is the distance between the gravitational centers of the masses.
Evolution = species change = observation = fact
How species change = Theory of Evolution = (doesn't lend itself to a quick equation) Variation in offspring, limited resources, differential reproductive success, propagation of successful traits.
The facts are facts, are based upon observations. The theories in both examples are subject to experimental analysis and can be disproven but can never be proven. A theory must explain ALL data, past present and future. A single data point not explainable by a theory disproves it and it must be modified or rejected.
If, for example, we found that objects in the Alpha Centari system (indulge my fantasy here) were attracted to each other by the cube of their distances we would have to reject the Theory of Gravity and come up with another that explains why it is the square in one region and the cube in another. This would not invalidate the fact, the observation here on Earth, just require another theory that explained both.
Likewise, if Darwin's theory of natural selection were proven wrong tomorrow, we would still be left with the fact of evolution. We would still have to explain HOW genetic traits change and adapt over time and how new species are produced.
I apologize for the redundant post, I think that some points need to be restated as many times as necessary to make them clear.
Edited by Lithodid-Man, : Changed "lengthy" to "redundant" in the last sentence
Doctor Bashir: "Of all the stories you told me, which were true and which weren't?"
Elim Garak: "My dear Doctor, they're all true"
Doctor Bashir: "Even the lies?"
Elim Garak: "Especially the lies"
™ Version 4.2