Historical Theology is the study of how Christians have understood various theological topics throughout history. The focus is on how God interacted with humanity and what he taught at a specific time in Scripture, but also how Christians understood any of those topics at different points in history.
One of the benefits of this approach is that it highlights the mistakes of Bible characters and other believers throughout Christian history made by studying Scripture over the centuries. Systematic Theology sometimes draws from this historical information but uses other sources as well. Biblical Theology takes a focus on a specific time of understanding rather than including the whole biblical picture.