The term theology is derived from the Greek words theos (meaning God) and logos (meaning speech or reason). B. B. Warfield’s short definition was simply: “Theology is the science of God and his relationship to man and the world.” Dr. Charles Ryrie states: “Theology simply means thinking about God and expressing those thoughts in some way.” Christian theology is Biblical meaning that it is primarily based on the canonical Scriptures of the Old and New Testaments. That certainly should not be taken to mean that the Scriptures are not viewed critically. I believe a critical view is very important for good exegesis but that is a topic for another post.
Systematic theology, more fully defined is the discipline that presents a coherent or harmonious view of the entirety of Biblical truth relating the various books to one another rather than simply using individual texts isolated from one another. Systematic theology then applies those truths the lives of people. Systematic theology is contemporary thus prompting the need for each generation to develop its own. Systematic theology can be said to summarize doctrine as it should be understood by present-day Christians. Biblical truth certainly does not change but the application of that truth in a dynamic, changing world is vital.
Systematic theology is but one approach to theology. Others include biblical, historical, and philosophical theology. Biblical theology concerns itself with the progressive revelation of God’s word recognizing that God did not reveal Himself all at once but rather over a long period of time through many different people. Biblical theology pays attention to the lives of the writers and how they and those who received their writings understood them in their time. Systematic theology makes use of Biblical theology and often builds on the results.
Historical theology is concerned with what those who studied the Bible in the past thought of the teachings. This can be individually or collectively. Historical theology helps the Bible student of today reach their own understanding of God’s truths by knowing how the contributions and even the mistakes made in the history of the church.
Philosophical theology provides a method of critically analyzing the Biblical text, Christian insights, doctrines, etc. For the Christian, the philosophical theology helps us understand the Bible as it understands itself (no small feat!) and also serves (at times) to aid the theologian in proving the points in his/her particular theology.
 Erickson, MIllard J. Christian Theology 2nd Edition. (Grand Rapids: Baker Academic, 1998), 22-23.