The name Bipolar came from the fact that the device operation depends on movement of charge carriers with both polarities (holes and electrons).
BJT has three terminals a) Emitter b) base c) collector
Emitter is heavily doped collector is moderately doped and base is lightly doped as we need most of the current from emitter should reach collector; base current is needed only as a pilot signal for collector current variation.
Transistor is active device which has the ability to control the electron flow through it.
BJT is of two types 1) PNP transistor 2) NPN transistor .
PNP transistor is formed by sandwiching n-type semiconductor between two P-type semiconductors.
Similarly PNP transistor is formed by sandwiching n-type semiconductor between two P type semiconductors.