A contactor is an electrically controlled switch used for switching an electrical power circuit. They are used to control electric motors, lighting, heating and other electrical loads.
Basic contactors have three components: the contacts, which carry current and include power contacts, auxiliary contacts and contact springs; the electromagnet, which provides the force to close the contacts; and the enclosure, which houses the mechanism.
Contactors come in many forms with different capacities and features. Typically, they are used to switch a circuit with a much larger power level than the control circuit - a 24 volt contactor controlling a 220 volt motor switch, for example. They cover a huge range of breaking current and can switch circuits up to several kilovolts.