Graeme Goldsworthy, in his excellent book According to Plan: The Unfolding Revelation of God in the Bible, provides a few brief, but helpful summaries of the various branches of theology. Make note of these summaries since they will likely prove to be useful in helping someone else understand the different theological disciplines.

Systematic theology asks:

What should Christians believe now about any aspect of Christianity? Its results: Christian doctrine.

Historical theology asks:

What have Christians believed about their faith at any given time? Its results: A record of the development of Christian doctrine.

Pastoral theology asks:

How should Christians minister to one another so that they grow to maturity in Christian living? Its results: Care and growth in the local church.

Biblical theology asks:

By what process has God revealed himself to mankind? Its results: The relating of the whole Bible to our Christian life now.