A relay is an electromechanical switch that uses an electromagnetic coil to open or close a set of contacts. It operates based on the principle of electromagnetism, where a small current flowing through the coil generates a magnetic field that attracts a movable armature. This armature either completes or interrupts a circuit, allowing for the control of high currents and voltages without needing direct human intervention. Specifically, a 12V relay is designed to operate with a 12-volt DC power supply, making it suitable for various automotive and electronic applications.