An NPN transistor is a type of bipolar junction transistor (BJT) that consists of three layers of semiconductor material: n-type, p-type, and again n-type. The three layers are called the emitter, base, and collector. In an NPN transistor, the flow of current is primarily from the emitter to the collector, with the base controlling this flow. NPN transistors are widely used in electronic circuits for amplification and switching applications. When a small current flows into the base, it allows a larger current to flow from the emitter to the collector, making them essential components in devices like amplifiers and digital logic circuits.