Rectifier is an electrical device that converts alternating current (AC) into Pulsating direct current (DC). This process of conversion of alternating current (AC) to direct current (DC) is called rectification.
There are two basic types of rectifier on the basic of functionality i.e., half wave rectifier (simplest rectifier with a single diode), full wave rectifier (either uses two diodes and a transformer [centre tap rectifier] or uses four diodes [bridge rectifier]).
With advent of power electronics these rectifiers further can be classified in many ways, diodes can be replaced by power diodes, OFF-ON cycles can be customized and power handling capacity of these devices can be improved.