What is the primary difference between an NPN and a PNP transistor?

An NPN transistor has a thin p-type layer between two n-type layers. A PNP transistor has a thin n-type layer between two p-type layers.

Because of this, the main charge carriers and the direction of current flow are opposite. In an NPN, current flows into the collector and base. In a PNP, it flows out of them.