A Bridge Rectifier is a very popular and widely used circuit. The Rectifier converts an AC supply into a DC supply. The circuit requires only four diodes.
The four diodes operate two at a time. That is, during the positive half cycle, diodes D1 and D4 are in forward bias and conduct. Diodes D2 and D3 are in reverse bias. 
In the negative half cycle of the AC supply, diodes D2 and D3 are in conduction while D1 and D4 are in reverse bias.
Thus the current from the rectifier flows in only one direction.
Bridge rectifiers are used in Alternators for excitation of the field. They are also used in welding machines for DC welding and in battery Chargers
Bridge Rectifiers are also used in measurement circuits to measure the amplitude of an alternating signal.
Controlled rectifiers contain thyristors instead of diodes. This enables control of the output supply.