This picture shows an internal view of a relay. The photo was made by scanning a the relay at a resolution of 600 DPI.
Relays are often used to use a small amount of power to control devices that require larger amounts of power. Relays operate using an electromagnetic coil and a switch. When current is applied to the coil the resulting magnetic field pulls the switch closed allowing current to flow through the switch. There is no connection between the coil and the switch which means that any device that supplies the power to close the relay will not be affected by the current that passes through the switch.