As I was reflecting on a conversation I had today, I realized there are a great many people (particularly Christians) who don't know what theology actually is. Some view it as an ivory tower activity reserved for the academic elite, other's seem to think that any semi-spiritual idea counts.
The word "theology" comes from the joining of the Greek words "theos" which means God, and "logos" which means word, study, or logic. So "theology" is "The study of God." In broader terms, it is the systematic study of religion and/or religious teaching.
Lest you think this only relates to academics, let me make my case for why you should care about theology. Christianity is a knowledge tradition. Our faith is one rooted in teaching, in the truth of certain facts about reality. These facts are susceptible to two sorts of attacks: Denial and Distortion. Some people (say, Stephen Hawking, Michael Ruse, and Richard Dawkins) deny the truth of religious teachings. Other people (false teachers like Mary Baker Eddy) distort certain religious teachings (and may deny others).
As Christians, it is vital for us to understand what we believe and why it is worth believing. Only when we do this can we stand against attacks of the sort mentioned above. Additionally, as Tozer said, we will never live above our conception of God. The vibrancy of our Christian life will be tied to our understanding of the teachings of Christ as revealed in the Bible. As Tozer also said, nothing less than a whole Bible can make a whole Christian.
Know thy doctrine.