Relays are switches that open and close circuits electromechanically or electronically. They control one electrical circuit by opening and closing contacts in another circuit.
Relays are generally used to switch smaller currents in a control circuit and do not usually control power consuming devices directly. Nonetheless, relays can control larger voltages and amperes by having an amplifying effect: a small voltage applied to a relay can result in a large voltage being switched by the contacts.
There a several types of relays that can perform a wide variety of functions in industrial applications: from preventing equipment damage to providing switching capabilities for heavy duty equipment.