A relay is a device which is used to turn on and off a heavier load from a smaller signal.  They are traditionally an electro-mechanical device, although solid state semiconductor relays are also popular. One of the key characteristics of relays is that there is a physical seperation between the activating signal and the load power.  For example a small battery can energize a relay to turn on a large wall-powered AC load such as an electric motor.  

We stock a large variety of both electro-mechanical relays and solid state relays.  For contactors and and starters specifically designed for starting electric motors, please see our Motor Contactors and Starters collection.