I found this very nice entry at the MadSci Network in which Dan Berger offers this:
The current consensus among philosophers of science seems to be this:
Laws are generalizations about what has happened, from which we can generalize about what we expect to happen. They pertain to observational data. The ability of the ancients to predict eclipses had nothing to do with whether they knew just how they happened; they had a law but not a theory.
Theories are explanations of observations (or of laws). The fact that we have a pretty good understanding of how stars explode doesn't necessarily mean we could predict the next supernova; we have a theory but not a law.
This is not, "a simplistic, hierarchical view of the relationship between theories and laws whereby theories become laws depending on the availibility of supporting evidence." (from the same site)
If you're an auditory learner and need a nice, simple explanation try this page, scroll to the bottom and click on, "To hear this program click here." From the transcript:
D: Well, the definition of a law is easy. It's a description--usually mathematical--of some aspect of the natural world.
Y: Like gravity.
D: Exactly. The law of gravity describes and quantifies the attraction between two objects. But the law of gravity doesn't explain what gravity is or why it might work in this way. That's because that kind of explanation falls into the realm of theory. And the theory that explains gravity is the theory of general relativity.
Y: Right. According to the National Academy of Sciences, a scientific theory is a "well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses." In other words, all scientific theories are supported by evidence, and you can test them, and--most importantly--you can use them to make predictions.
D: So based on that definition, theories never change into laws, no matter how much evidence out there supports them. Formulating theories, in fact, is the end goal of science.
Y: So to say evolution is just a theory is actually an argument for it and not against it. You can't do any better in science than to be a theory.
Here's a fun page from high school chemistry teacher, David Dice - Proof and Science. This page isn't simply valuable for the person who doesn't understand the difference between theory and law but also for those of us who tend to talk of evolution being 'true'.
No scientist will ever claim that a theory is true. What they will do is state that the evidence agrees with the theory.
Maybe those of us who accept the theory of evolution need to tidy up our terms too. The site is an excellent read, generally agrees with the definitions from the MadSci Network and has a quiz. Kudos Mr. Dice!
Let's hear from Ronald Matson, Professor of Biology at Kennesaw State University but instead of his definition of laws and theories (you can guess by now what at least one of his definitions will be), let's hear what he has to stay about what laws and theories most definitely are not:
Regardless of which definitions one uses to distinguish between a law and a theory, scientists would agree that a theory is NOT a "transitory law, a law in waiting". There is NO hierarchy being implied by scientists who use these words. That is, a law is neither "better than" nor "above" a theory.
I hope this clears the matter up somewhat. A theory is not a baby law. A law is not a theory that's been proven. Next time you're discussing the matter with someone defending the theory of evolution you will know that you simply can't, with any honesty, claim evolution is a theory because it's unproven. Next time you're discussing the matter with a creationist you will know that when you insist evolution is the truth, you're using inexcusably sloppy language.