What are relays?

Answer 1

A relay is a electro-mechanical switch. It uses electro-magnetism generated from a small voltage/current (for normal circuits its typically around 5V/6V/12V, 200mA) to switch larger voltages/currents (around, 110V/220V,16A) for household appliances.

(OR)

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 low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations.