Theories are descriptions, and they come in two flavors: true and false. So the Theory of Evolution can be both a theory and true, which is just what a great number of scientists believe. When evolution by natural selection is called a theory, however, this is sometimes intended to emphasise that there is no proof that it is true. Now if by 'proof' we mean what pure mathematicians produce, then this is correct. There is no proof of the Theory of Evolution, and there is no proof of any other empirical theory either. Proof in this sense is not an option in science, because all theories go beyond the evidence upon which they are based. There can similarly be no proof that the sun will rise tomorrow. But the sense in which it is true that there is no proof of evolution is compatible with the claim that there is overwhelming evidence that it is true, which is again what a great number of scientists believe.
Read another response by Peter Lipton
ID theorists and creationists like to say that the Theory of Evolution is "just a theory." Is that true? What does that mean? What's the difference between "truth" and "theory"?