Hii

Transistor is a three terminal device formed either by sandwiching N type between 2 P type semicondutors or P type between 2 N type semicondutors. The transistor has 3 parts - emitter which is thick and heavily dopped, Base which is thin and lightly dopped, and the Collector which is again thick and heavily dopped
The transistor has Three stages- 
Cutoff state :
It is the state of the transistor when the input voltage is less than potential barrier and transistor is not conducting. 
Active State :
It is the state of the transistor when the input voltage becomes greater than the potential barrier and the emitter current increases which in turn increases the collector current.
Active state is used in the transistor as amplifier .
Saturation State :
It is the state of the transistor when the output current i.e. the collector current becomes constant. 
The cutoff state and saturation state can be used in transistor as switch.

Hope this helps
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