The operation of an electromechanical relay is based on electromagnetism. When current flows through the relay coil, the coil produces a magnetic field.
The coil is usually made from wire wrapped around an iron core. When the coil is energized, the magnetic field becomes strong enough to attract a movable iron piece called the armature.
The armature is normally held in its resting position by spring tension. When the magnetic force from the energized coil overcomes the spring, the armature moves.
As the armature moves, it changes the position of the relay contacts. These contacts are what open, close, or transfer electrical circuits.
Normally open contacts are open when the relay is de-energized. When the relay picks up, the normally open contacts close and allow current to flow through that circuit.
Normally closed contacts are closed when the relay is de-energized. When the relay picks up, the normally closed contacts open and interrupt that circuit.
Changeover contacts transfer a circuit from one path to another. When the relay changes state, the common contact moves from the normally closed side to the normally open side.
When current is removed from the relay coil, the magnetic field collapses. The spring returns the armature back to its normal resting position.
This returns the relay contacts to their normal state. Normally open contacts open again, normally closed contacts close again, and changeover contacts return to their original position.
Relays are useful because a low-power control circuit can operate a separate circuit that may carry higher voltage or current. This provides electrical isolation between the control side and the contact side.