Relay

A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. Relays are found hidden in all sorts of devices. In fact, some of the first computers ever built used relays to implement Boolean gates.

An open relay

Reference: http://www.howstuffworks.com/

Mechanical Relay

Mechanical Switching - Relays
A relay consists of an electromagnet (a coil around a iron bar) that can automatically cause one or more switches to move between to contacts. When current is applied to the electomagnet the switch moves towards the magnet, otherwise the switch is in its "normal" position. By wiring one of the switches into the electromagnetic circuit the relay can be made to remember and thus act as a storage device. In general a relay can be used to make other switches in second and third circuits to move.



From http://www.howstuffworks.com

A relay is so-called since it "relays" or copies an action at one point to another. With simple modifications a relay can duplicate a single action into multiple copies, and even remember an action, thus creating a simple memory device.