[Al Skierkiewicz]

An SCR is a silicon controled rectifier. This particular type of device can act as a switch and can be turned on with a very low control voltage but as it's name implies (and it's schematic symbol shows) it can only control current in one direction. It's other failure is once turned on, it cannot be turned off without removing the power that it turned on. This charachteristic while it sounds bad works very well in controlling pulsed DC power or in acting as a phase controled switch on AC power. It actually makes an interesting power supply when used in a bridge configuration. It's AC version is a triac and those are sued in virtually every light dimmer in use today.