Controlled Rectifiers are rectifiers which have thyristors in place of a diode. A thyristor is a three terminal device which can be switched on by applying a suitable gate voltage. 
In a Controlled Rectifier, generally two of the diodes are replaced with a thyristor. The thyristor enables the enables switching on the output of the rectifier at the desired time. This allows the output voltage and current to be controlled.
A rectifier with two thyristors is called a half controlled rectifier while a rectifier where all the diodes have been replaced with thyristors is called a fully controlled rectifier.
Controlled Rectifiers are also known as converters.