In an NPN transistor, the base controls the amount of current passing through it while allowing current flow from the emitter to the collector. PNP transistors, on the other hand, are made to flow current from collector to emitter.
Three separate layers of semiconductor material are stacked together to create transistors. Doping is the process of adding extra electrons to some of those layers while depleting electrons from others (doped with "holes" — the absence of electrons). Because electrons have a negative charge, a semiconductor material with additional electrons is known as an n-type, while one with excess electrons removed is known as a p-type (for positive). Either a n on top of a p on top of a p, or p over n over p, is used to build transistors.
The goal of the NPN transistor is to allow electrons to move from the emitter to the collector (so conventional current flows from collector to emitter). The base, which regulates the number of electrons the emitter "emits," receives electrons from the emitter. The collector "collects" the majority of the electrons released and passes them on to the circuit's subsequent component.
A PNP operates in a similar but reverse manner. The base still regulates the current flow, but it now goes from emitter to collector in the reverse direction. The emitter emits "holes" (a hypothetical absence of electrons), which are gathered by the collector, rather than electrons.
Hence, in NPN transistors, the base regulates the amount of current passing through it while allowing current to flow from the emitter to the collector. PNP transistors, on the other hand, are made to allow current to flow from the collector to the emitter.
