The National Academy of Sciences has some great new material that may come in handy if you ever get into a discussion with proponents of so-called "creation science" or "intelligent design." Science, Evolution, and Creationism is a free 88-page PDF e-book, and there is also an 8-page brochure that provides a quick summary of the key points.
One of the things that seems to confuse people about evolution (and science in general) is the word "theory." It has a very specific meaning in the context of science, and it doesn't mean the same thing as in common conversation. If you have a theory about who has been eating your secret stash of cookies at the office, it means a sort of a hunch, or perhaps an informed guess. As explained in the NAS brochure, a scientific theory is quite different: "In science, the word theory refers to a comprehensive explanation of an important feature of nature that is supported by many facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena."
The theory of gravity is such a theory - it doesn't mean that scientists have a hunch that there might be gravity. It means that they understand gravity in a very detailed and systematic way, and they can use this understanding to (for example) design spacecraft trajectories. Scientific theories can change over time as new observations and experiments are done. Newton's theory of gravity provides an extremely good approximation that still works well for most purposes (including satellite operations). Einstein's General Theory of Relativity confirms the predictions of Newton in most respects, but also accounts for some usually small effects that Newton's theory cannot (sometimes these effects are not small, e.g., black holes).
The theory of evolution is similarly well understood so it can be used to explain observations in the fossil record, Earth's biological diversity, changes in DNA among different organisms, and practical applications such as the ability of bacteria to develop resistance to antibiotics. Evolution is an observed fact. The theory of evolution is a comprehensive explanation of how evolution works and forms the core of modern biology and medicine (with applications in other fields too). The fact that this is even up for debate among educated people in the United States is a bit embarrassing, but there you go (at least Huckabee seems to be out of the running in the presidential race). This new material is a good resource for when this does come up.