This instructable deals with electromagnetic relays. They are electromagnetic switches working based on the principle electromagnetic induction.
E-M Relays are used where any circuit with a different voltage level has to be controlled using a low voltage level circuit. Just like controlling a 230 VAC bulb with a 5 V DC output of a microcontroller.Relays play a very crucial role in many categories of electronic circuits.
For more information about relays, go through this.”