Apologetics is a subset of theology, concerned strictly with justifying the tenets of the faith to doubters and nonbelievers.

Thomas Aquinas, by contrast, argued for the existence of God only briefly at the very beginning of the Summa Theologica, and devoted the rest to elucidating the properties of God, the other supernatural beings, and humanity. Much of theology is philosophy done with some particular background assumptions; apologetics is argument and rhetoric in defense of those assumptions.

ETA: In the modern world, most of the positive arguments for the existence of God are (of course) fatally flawed. The older "mainline" denominations realize this on some level and have essentially fallen back to the position "You can't know that there's not a God", which is something of a defense against losing one's own faith but not a great opening gambit for winning converts. The newer Protestant denominations aren't generally aware of the flaws in their arguments, and so use them to win converts.

In particular, the mainline denominations (and their theologians) shy away from empirical tests, while the newer denominations (and their apologists) embrace bad empirical tests. This is of course an oversimplification, but it's generally true.