An electromechanical switch operated by a solenoid coil. Relays are commonly fitted in controllers as alarm/limit or time proportioning control outputs.
The limited current capacity and switching cycles of internal relays means that they are usually connected to larger external slave relays/contactors with much larger switching capability. These are easier and cheaper to replaced than an complete controller once worn out.
A suitably rated RC snubber should be connected to relays to protect nearby equipment from the effects of noise generated as they switch. The reliability and long-term cost savings of solid state relays make them more suitable than relays in many applications.