Relays are electrically operated control switches, and may be classified as either power relays or control relays, according to their use. Power relays are frequently called contactors. Control relays are usually known simply as relays.

As you can see in this illustration, there are four basic parts to the relay. The coil of wire becomes magnetized when a voltage or current is applied to it. The armature is connected to the contacts. (Generally, the armature and the contacts are electrically isolated from one another.) The spring keeps the relay in its normally open or normally closed position when no power is applied.

Excerpt from “Electricity for HVACR Technicians“, available from the online store at RSES.ORG.