A slight difference...

In science, a theory is a comprehensive explanation of observed phenomena which manages to cover all bases.  Its value can be seen when the scientific theory is used to predict events--the arrival of a comet, the shape of a hummingbird bill as derived from the shape of the flower it feeds upon, the existence of subatomic particles which explain the behavior of atoms--and are subsequently discovered.

A hypothesis is not a theory.  A scientific theory does not mean that someone has an opinion about something; it is a set of FACTS.  A hypothesis is a tentative explanation of observed phenomena that cannot attain the status of theory until the scientific method has provided enough evidence to support the explanation.  Only then does it become a theory, a term indicating its validity in the face of testing.

This means, of course, that the theory of evolution is not someone's opinion.  It is a set of facts.  The difference between science and other types of thinking is that science never claims to have discovered the ultimate truth, since as science itself advances in its ability to observe phenomena, new events may come to be known.  It is precisely science's astonishing ability to observe more and more of the cosmos that is so exciting and ground-breaking.

Nothing so far, however, has been able to undermine the facts on evolution.  Quite the contrary, it has been enlarged by many thinkers who add a swirl here, a squiggle there, to enrich it hugely.

So when someone tells you that evolution is "only" a theory, that person really means that it is just someone's opinion.  The person does not understand what a scientific theory is and cannot distinguish it from a hypothesis.  Most of these people, however, are uninterested in the difference, since their agenda is usually religious, where they want you to accept a given truth and stop thinking.

It would be well to note that Isaac Newton, one of the greatest scientists who ever lived, was a religious man, as were many others before and after him.  The difference, however, is that their God was impressive enough to work with atoms, subatomic particles, calculus, gravity, the Big Bang, eons of time, DNA, the primeval ooze, and the evolution of life.  Newton didn't go around scared silly of finding out that orbits are elliptical and Darwin wasn't appalled into silence at the idea that all life crawled out of some mudflat (a fact that DNA has amply proven).  Their God was not one of these prissy little man-made, small-brained creations that don't want you to think.

Their God was magnificent, flinging the universe down as a challenge, defying humankind to discover its fathomless secrets bit by bit, century by century, one atom at a time.  Now that was a God not made in man's image.