A bipolar junction transistor, or BJT, is a type of transistor used to amplify signals or switch current. It has three terminals, usually called the base, collector, and emitter.
How a BJT is controlled
A small current into the base can control a larger current between the collector and emitter. This makes BJTs useful in amplifiers, simple switches, signal circuits, and older logic designs.
BJT and other transistors
A BJT is one transistor family. A MOSFET is another common family, often preferred for efficient power switching. The better choice depends on the job.
In short, a BJT lets a small control current manage a bigger current.