The primary objection to evolution that is so often repeated by creationists is the old “it’s only a theory” canard. This little number relies upon a bit of dishonest wordplay. The term ‘theory’ has at least two distinct meanings: “1. a coherent group of general propositions used as principles of explanation for a class of phenomena: Einstein’s theory of relativity.” and “2. a proposed explanation whose status is still conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact.”  Creationists will try to replace the first definition with the second so that it would appear that evolution is conjectural (and therefore likely to be false).
When the term theory is applied to evolution it actually refers to the first definition. Gravity is only a theory too (there’s actually more evidential support for evolution than there is for the theory of gravity)! It would be useful for creationists to understand exactly what the word theory means in a scientific context, so I shall turn to a quote to explain it:
A theory is what one or more hypotheses become once they have been verified and accepted to be true. A theory is an explanation of a set of related observations or events based upon proven hypotheses and verified multiple times by detached groups of researchers. Unfortunately, even some scientists often use the term “theory” in a more colloquial sense, when they really mean to say “hypothesis.” That makes its true meaning in science even more confusing to the general public.
In general, both a scientific theory and a scientific law are accepted to be true by the scientific community as a whole. Both are used to make predictions of events. Both are used to advance technology. 
A theory is actually one of the highest orders in science. Some times theories are adapted or thrown out when new evidence comes to light, a strong theory is one that can last for centuries without being disproved. Evolution has been tested for 150 years, and any new findings have merely served to confirm the theory. As far as theories go evolution is extremely strong, and it explains a vast range of phenomena. It is far from vague conjecture.