In electronic parlance, a relay is a switch which is actuated by an electrical signal. Contemporary electronic relays fall into two main categories: electromagnetic, and solid state.

The classic electromagnetic relay is a switch which is thrown by an electromagnet. A relatively low current applied to the magnet can throw the switch, allowing a higher current to flow through that switch. The solenoid of most automobiles can be considered an electromagnetic relay.

In digital applications, it has been surpassed by the solid state relay. These relays have no moving parts, so they can switch very quickly in response to a control signal. They are built from semiconductors, and they cannot handle the current that an electromagnetic relay could, but their advantage is speed.

High current solid-state relays often require heatsinks to drain excess heat.