Relays are a type of switch that is operated electronically to turn on or off current within an electric circuit. Relays operate similar to a typical light switch, where power is switched on and off when the switch lever is moved, the main difference is that a relay is operated by an electrical circuit rather than manually pressed.
Relays have many different uses and are often utilised in situations where isolation is necessary or when a circuit needs to be controlled with a low-power signal. There are also specialised types of relay that are used for very high power applications, these are referred to as contactors.