Presuppositional apologetics argues that Christianity, the teachings of which come from the Bible, is the only legitimate foundation for rational thought.
It argues that every worldview has a first principle, or a foundational presupposition, which the rest of the worldview depends upon. Presuppositional apologetics begins with the presupposition that the Bible is divine revelation, and thus the source of all Truth, and it seeks to expose flaws with the presuppositions of every other worldview.
The primary difference between presuppositional and classical/evidential apologetics is that whereas classical/evidential apologetics seek to use evidence to demonstrate the truth of the Bible, presuppositional apologetics argues that the ultimate proof for the Bible involves demonstrating the necessity of biblical revelation for anything to make sense at all in the first place.
Presuppositional apologists recognize that there is some value to classical/evidential apologetics, namely, that the evidence does indeed point towards the truth of the Bible, but they would also argue that the most powerful argument for the Bible comes from the truth and necessity of the Bible itself, and not from evidence, science, or empiricism.