A relay is an electrically controlled switch. Electromechanical relays use an electromagnet to activate mechanical contacts, solid-state relays use semiconductor switches.
A latching relay stays in the current state when power is removed. A non-latching relay reverts to a known state when power is removed.
Relay contacts come in several varieties. The most common are:
A: normally open B: normally closed C: one normally-open and one normally-closed both actuated by a single coil X: normally open, double make Y: normally closed, double break Z: one X and one Y, both actuated by a single coil