An SCR blocks current in the reverse direction (cathode to anode) like a diode. It also blocks current in the forward direction (anode to cathode) until it receives a trigger pulse current applied to the gate terminal (-cathode, +gate). Once an SCR has been gated, it behaves like a diode and conducts current until it is forced to turn off (commutated) by some external event such as the reversal of the applied voltage. Since the AC line voltages reverses with every cycle, a rectifier circuit made with SCRs is naturally line commutated every time the cycle reverses.
The output voltage of an SCR circuit is adjusted by delaying the gate pulse after the point in the AC line cycle where a diode would start conducting. The longer the delay, the lower the output voltage.
One of the main uses for SCRs is to control the speed of DC motors. The converter and associated motor control circuitry is often called a DC drive or an SCR drive or even just an SCR. Converters are often used to dim incandescent lights and called light dimmers in that configuration.
SCRs are also called thyristors, but there are also other types of thyristors.
General Electric SCR Manual Fifth Edition 1972