Answer

As a somewhat rough analogy, the difference between an apologist and a theologian is like the difference between a science teacher and a scientist: Just as a science teacher explains science and a scientist develops human understanding of science, an apologist explains and defends what is known about the faith, while a theologian explores the faith and seeks to expand upon what is known about the faith. What that can mean in practical terms is that while a theologian should be prepared to act as an apologist, an apologist need not be a theologian.