Systematic theology is a branch of Christian studies that attempts to develop coherent systems of statements about various topics within the Christian faith. It is a broad study that comprises a wide range of beliefs and methodologies. In practice, it often combines elements of both biblical theology and philosophical theology.
Like biblical theology, systematic theology usually draws on biblical texts for most of its source material. One major goal of biblical theology is to trace the historical development of doctrines and ideas in the Bible, especially progressive revelations about the nature of God throughout the Bible. Systematic theology, on the other hand, tends to focus more on individual topics than on the historical development of thinking about those topics.
For instance, the goal of a particular work of systematic theology might be to explicate biblical statements regarding redemption. Many different passages in the Bible deal with the concept of redemption, but few, if any, systematically define and explain redemption. A systematic theologian might compare and analyze all of these various texts with the goal of defining a system of redemption that takes all of them into account.
Sometimes, however, the attempt to reconcile various texts may draw the systematic theologian outside of what is explicitly stated in the Bible. For instance, many, if not most, views of Christian theology hold to some form of Trinitarian doctrine — the belief that the Father, Son and Holy Spirit are three persons who comprise a single God. While there is some biblical support for this doctrine, the word "Trinity" itself never appears in the Bible. In order to defend or refine doctrines relating to the Trinity then, systematic theology will most often branch into the field of philosophical theology. Philosophical theology uses the rules of logic, rather than explicit biblical teaching, to formulate statements about the nature of God.
Other theological viewpoints might also fall under the broad umbrella of systematic theology. For instance, process theology is a systematic theory that postulates that both God and humanity change over time. Some critics of process theology believe that it outright contradicts biblical teaching, which is generally not allowable in systematic theology.