Relay is electrical device (circuit breaker) designed for opening and closing various portions of electrical circuits for given changes in electrical or non-electrical input quantities.
Relay types can be distinguished by the control signal and execution, we will not dwell on it, it’s all there on the same Wikipedia. We only note that the most widely used electrical (solenoid) relay.
To understand why we need from the definition of difficult is switch, so explain in the simple words:
The relay is designed for switching of a large currents of load. In other words, it is a switch, and is even easier – the principle of the relay – low current (eg signal button) turn on a chain with a large shock. A relay is used when an actuator (starter, alternator, fan, mirror, horn, etc.) consume more current (30-40 amperes).