I'm going to start another regular series of posts (like the series titled Biblical Wisdom), but this series will be devoted to clearing up the confusion and refuting the tired old criticisms... so in the future I can just say, see Part x.
First up, let's tackle the tired old "Evolution is only a theory" type of statement.
This is the "argument" that some creationists try to use to deflect the mountain of scientific evidence that backs up the theory of evolution. Oftentimes the offending person will add something like "It isn't truth, only a theory" or "Evolution is a theory not a fact." The reason that this lame "attack" sometimes works is that the vast majority of the public does not understand what a scientific theory actually is. Some of us "remember" learning back in school that a theory is not a certainty--it is more than a hypothesis but below a law.
Scientists do not use the terms that way. According to the National Academy of Sciences (NAS), a scientific theory is "a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses." There is no "magical" point at which a theory becomes a law. In point of fact, the term "law" is simply a descriptive generalization about nature.
Again, turning to the NAS, a fact is defined as "an observation that has been repeatedly confirmed and for all practical purposes is accepted as 'true.'" The fossil record and abundant other evidence testify that organisms have evolved through time. Although no one observed those transformations, the indirect evidence is clear, unambiguous and compelling. All sciences frequently rely on indirect evidence. Physicists cannot see subatomic particles directly, for instance, so they verify their existence by watching for telltale tracks that the particles leave in cloud chambers. The absence of direct observation does not make physicists' conclusions less certain.
When scientists talk about the theory of evolution (or the atomic theory, the theory of gravity, or the theory of relativity) they are not expressing reservations about its truth. Just the opposite.
Furthermore, it is necessary to define the word "evolution." From the archives of TalkOrigins.Org: Like so many other words, it (evolution) has more than one meaning. Its strict biological definition is "a change in allele frequencies over time." By that definition, evolution is an indisputable fact. Most people seem to associate the word "evolution" mainly with common descent, the theory that all life arose from one common ancestor. Many people believe that there is enough evidence to call this a fact, too. However, common descent is still not the theory of evolution, but just a fraction of it (and a part of several quite different theories as well). The theory of evolution not only says that life evolved, it also includes mechanisms, like mutations, natural selection, and genetic drift, which go a long way towards explaining how life evolved.
Calling the theory of evolution "only a theory" is, strictly speaking, true, but the idea it tries to convey is completely wrong. The argument rests on a confusion between what "theory" means in informal usage and in a scientific context. A theory, in the scientific sense, is "a coherent group of general propositions used as principles of explanation for a class of phenomena" [Random House American College Dictionary]. The term does not imply tentativeness or lack of certainty. Generally speaking, scientific theories differ from scientific laws only in that laws can be expressed more tersely. Being a theory implies self-consistency, agreement with observations, and usefulness. (Creationism fails to be a theory mainly because of the last point; it makes few or no specific claims about what we would expect to find, so it can't be used for anything. When it does make falsifiable predictions, they prove to be false.)
Lack of proof isn't a weakness, either. On the contrary, claiming infallibility for one's conclusions is a sign of hubris. Nothing in the real world has ever been rigorously proved, or ever will be. Proof, in the mathematical sense, is possible only if you have the luxury of defining the universe you're operating in. In the real world, we must deal with levels of certainty based on observed evidence. The more and better evidence we have for something, the more certainty we assign to it; when there is enough evidence, we label the something a fact, even though it still isn't 100% certain.
What evolution has is what any good scientific claim has--evidence, and lots of it. Evolution is supported by a wide range of observations throughout the fields of genetics, anatomy, ecology, animal behavior, paleontology, and others. If you wish to challenge the theory of evolution, you must address that evidence. You must show that the evidence is either wrong or irrelevant or that it fits another theory better. Of course, to do this, you must know both the theory and the evidence.
So the next time somebody says that evolution is just a theory you can smile and know that they are not well informed.