Relays, simply put, are electric switches that we can control using a small output signal. They're perfect for small controllers, like Raspberry Pi, to switch other circuits with higher voltages on and off. The simplest example is to use a relay to switch a lamp on and off. We can't light the lamp using the voltage available to us in Raspberry Pi, but we can use a relay as a switch to control the lamp.

The principal part of a normal relay is a coil. When electricity runs through it, it magnetizes an iron core, which in turn moves a lever from the Normally Closed (NC) connector to the Normally Open (NO) connector. When electricity is cut, a spring returns the lever from the NO connector to the NC connector. This movement of the lever from one connector to the other causes a characteristic clicking sound. This tells you that the relay works. The lever in turn is connected to the Common Ground (COM) connector.

The following figure illustrates how a simple relay is constructed. We control the flow of the current through the coil (L1) using our output SIGNAL (D1 in our case). Internally, in the relay, a resistor (R1) is placed before the base pin of the transistor (T1), to adapt the signal voltage to an appropriate level. When we connect or cut the current through the coil, it will induce a reverse current. This may be harmful for the transistor when the current is being cut. For that reason, a fly-back diode (D1) is added, allowing excess current to be fed back, avoiding harm to the transistor: