Evolution is often criticized (by those who have no idea what there talking about) that it is just a theory. This criticism is based a misunderstanding of what a scientific theory is. In science, a theory is not a guess or even an educated guess but a heavily tested explanation for an observation or a grope of related observations.
For a theory to become theory it starts out as a hypotheses. A hypothesis is a falsifiable explanation for an observation, it does not explain what it is but why it is. The scientists use that hypotheses to make testable predictions (making it falsifiable) (”If ‘X’ hypotheses is accurate than we should see ‘Y’ at ‘Z’” or “if ‘X’ hypotheses is accurate than we should see ‘Y’ if we do ‘Z’”) than they test those predictions. If the predictions are accurate than they/he/she publish there hypothesis in a pear reviewed journal. Then other scientists examine the hypothesis, predictions and experiments and repeat the experiments. If they find a flaw in the hypothesis, predictions or experiments the hypothesis is ether altered or dropped, if the hypothesis is found to be accurate than it becomes a theory or is integrated into a larger theory containing multiple related hypothesise.
A they isn’t just a guess, its heavily tested by many scientists and they don’t stop testing it. Theory’s are always being reviewed in the light of new theory’s and observations and cause of that theory’s are constantly being tweaked to match new evidence.
“But what about a scientific law? Doesn’t it go hypotheses, theory then law?”
No. A scientific law is different than theory’s and hypothesise.
Law are only used in physics and where a theory is an explanation for an observation a law is an observable phenomena put in concise terms (an action or set of actions), scientific laws are accepted to be true and universe. A good example of a law is the law of gravity; the law of gravity describes what gravity is, where the theory of gravity describes why it is.

Further reading:
http://en.wikipedia.org/wiki/Theory#Science
http://en.wikipedia.org/wiki/Hypothesis#Scientific_hypothesis
http://wilstar.com/theories.htm
http://www.jcu.edu.au/studying/services/studyskills/scientific/method.html
http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec01.html

Updated 3/12/08