At face value, a philosopher is a lover of wisdom. The Greek words philos and sophia mean "love" and "wisdom" respectively, and this is certainly the kind of philosopher Socrates tried to be. Ever since the Enlightenment, however, academics have been trying to overcomplicate things, and for most secular thinkers a philosopher is one who attempts through his own reason and observation of the world to construct a complete, coherent, and consistent explanation and theory of all things.
A theologian, on the other hand, is exactly what he sounds like—one who studies God. Men at various times have attempted to deter theology from this noble course (particularly the German liberals in the 18th and 19th centuries), and in fact there are still those who call themselves "liberal theologians," but they don't really study what they claim to. Liberal theology is more concerned with man than God. Real theologians, however, reflect on God to learn more about Him and about His creation (most notably, mankind).
Ever since Tertullian (and probably before), Christians have argued about the relationship of theologians to philosophers. is there such a thing as a Christian philosopher? Can there be? In the secular sense, the answer is clearly no. Everything we can or need to know about life and the cosmos God has revealed to us, either through His Word or generally through His creation. Does this mean every Christian thinker who isn't a scientist or mathematician is a theologian? Was Soren Kierkegaard a theologian or a philosopher? What about St. Augustine, or C.S. Lewis?
Without getting into the doctrinal implications of such a statement, it might be easier to think of philosophers and theologians in terms of prophecy. Not all prophecy is concerned with telling the future: plenty (perhaps even most) is simply the illumination of difficult concepts or a preaching of the truth in response to heresy or immorality. In this sense, anyone defending God's truth against man's, anyone using divine revelation as found in the Bible as his standard for all things, is a prophet; secular philosophers, then, are false prophets.
Christians don't need to invent their own explanation for anything. God has given us His, and that is enough. That doesn't mean we can't deepen our understanding—we can, and should. We do need to compare everything to Scripture, however, and that is what Christian thinkers can help us do better than just about anyone else. Having some knowledge of their lives isn't absolutely necessary for understanding their ideas, but to see how pious Christian thinkers were, or how wicked their secular counterparts, can offer important insights on our quest for truth.