This is an old revision of the document!
Relays provide heavier duty switching (on/off) when it is impractical to switch directly.
A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal. – Wikipedia1)
Common uses:
Normally it takes power from the trigger source to operate the relay. A latching relay, however, moves from position to position and locks there even when power is removed.