The NPN transistor's ability to control a larger current (collector current) with a smaller current (base current) makes it a fundamental component in electronic circuits.
What is transistor?
The transistor functions as an amplifier when it is in the active region. Because of forward bias, a small current flows from the emitter to the base (EB junction), and because of reverse bias, a larger current flows from the collector to the base (CB junction).
Both of the junctions (EB and CB) are forward-biased in the saturation region. This happens when the transistor reaches its maximum current-carrying capacity, which happens when the base current is sufficiently increased.
Both of the junctions are reverse-biased in the cutoff region. This happens when the transistor is effectively turned off and there is no base current. There is very little collector current flowing, and the transistor is not conducting.