What is Relay ? | Relay working principle | Latching Relay, Solid State Relay, Thermal Relay

Relay is an electronic component used mostly in large equipment. Sometimes some work has to be done automatically in the circuit, based on different situations, at that time our relay is needed.
What is Relay?
Working Principle of relay ?
Working Example of relay ?

Poles and Throws
1. Single Pole Single Throw
2. Single Pole Double Throw
3. Double Pole Single Throw
4. Double Pole Double Throw

Types of relays and there working principle
1. Latching Relay
2. Solid State Relay
3. Thermal Relay
4. Reed Relay