What is (1) active region (2) cut-off region (3) saturation region?

Explanation:

Active region is that region in which emitter base junction is forward bias while collector base junction is reverse bias.This is the region in which transistor tends to behave as an open switch. The transistor has the effect of its collector and base being opened. This is the region in which transistor tends to behave as a closed switch. The transistor has the effect of its collector and Emitter being shorted. The collector and Emitter currents are maximum in this mode of operation. The figure below shows a transistor working in saturation region.