From an interview with Vern Poythress:
Different people have had different conceptions of both biblical theology and systematic theology, so it is wise to ask what people mean in both areas, as well as to look at the relation between the two areas.
I would myself describe systematic theology as study of the Bible’s teaching in which we try to synthesize and then summarize what the Bible as a whole teaches about all kinds of topics—God, man, Christ, sin, salvation, and so on.
In some contexts the expression “biblical theology” simply means theology built on the Bible; that is, it is systematic theology done in the right way. But there is also another possible meaning. Biblical theology, as described by Geerhardus Vos, studies the Bible with a focus on its history, the history of revelation and of redemption. Whereas systematic theology is topically organized, biblical theology is historically organized. It looks at the progress of God’s work and his revelation through time. In addition, biblical theology more broadly conceived can study the themes that are distinctive to a particular book of the Bible, or to books written by a single human author (for example, Paul’s letters).
At their best, biblical theology and systematic theology interact and help to deepen one another.
Systematic theology provides doctrines of God’s sovereignty, of revelation, of God’s purposes, and of the meaning of history that supply a sound framework of assumptions for the work of biblical theology.
Biblical theology at its best deepens the appreciation that systematic theology should have for the way in which, in interpreting individual texts and in uncovering their relation to a whole topic, the context of texts within the history of redemption colors the interpretation. Biblical theology may also bring to light new themes that can be the starting point for systematic-theological explorations into new topics that can receive fuller attention. For instance, the theme of life and death as it develops in the course of the history of revelation can become the starting point for discussing ethical questions about modern medicine and the issue of euthanasia.
For more on this, see Poythress’s essay, “Kinds of Biblical Theology.” For a survey of the relationship as understood in 20th century Reformed theology, see Lee Irons, “Biblical and Systematic Theology: A Digest of Reformed Opinion on Their Proper Relationship.”