Dr. Faustus is based on the legend of Faust. Dr. Faustus was a scholar who found that the conventional road to knowledge wasn't enough, so he made a deal with the devil for magical powers. (He swaps his soul for 24 years of service by a demon.) The play follows Faustus' adventures with his magic, then moves to his death, 24 years later, when he tries to avoid his damnation…but too late.
For a more detailed summary, see the enotes study guide, which discusses the play a scene at a time.