Why A bipolar junction transistor is built from three doped semiconductor regions: the emitter, the thin base in the middle, and the collector. A small current into the base controls a much larger current flowing between emitter and collector, which is how the device provides current gain. Field effect transistors use different names for their three electrodes: gate, source and drain.
Watch out Terms like input, output and supply describe roles in a circuit, not the physical electrodes of the device; any transistor terminal can serve as an input or output depending on the amplifier configuration.
BJT = Base, Emitter, Collector. FET = Gate, Source, Drain. Base and gate are the control terminals.