I have found the follow quotations very helpful. Both are from Geerhardus Vos and the full articles from which they appear are freely available online.
"In general, then, Biblical Theology is that part of Exegetical Theology which deals with the revelation of God. It makes use of all the results that have been obtained by all the preceding studies in this department. Still, we must endeavor to determine more precisely in what sense this general definition is to be understood. For it might be said of Systematic Theology, nay of the whole of Theology, with equal truth, that it deals with supernatural revelation. The specific character of Biblical Theology lies in this, that it discusses both the form and contents of revelation from the point of view of the revealing activity of God Himself. In other words, it deals with revelation in the active sense, as an act of God, and tries to understand and trace and describe this act, so far as this is possible to man
and does not elude our finite observation. In Biblical Theology both the form and contents of
revelation are considered as parts and products of a divine work. In Systematic Theology these same contents of revelation appear, but not under the aspect of the stages of a divine work; rather as the material for a human work of classifying and systematizing according to logical principles. Biblical Theology applies no other method of grouping and arranging these contents than is given in the divine economy of revelation itself."
The Idea of Biblical Theology as a Science and as a Theological Discipline.
http://www.biblicaltheology.org/ibt.pdf
and
"Systematic theology deals with the revelation of God. But systematic theology deals with it not as a process of divine activity in history; here revelation appears as a finished product, to be logically apprehended and systematized....Biblical theology, rightly defined, is nothing else than the exhibition of the organic progress of supernatural revelation in its historic continuity and multiformity"
The Nature and Aims of Biblical Theology
http://www.biblicaltheology.org/nabt.pdf
I would encourage you to read the articles (the second is shorter and easier) for a better view of what Vos is doing beyond these "sound-bite" definitions.