The control valve that releases the hot water is on the top of the sink near the faucet.
The small, metal water-heater tank is beneath the sink. It contains an electric heating element.
A small copper tube supplies water to the water heater from the household plumbing system.
A small vinyl tube is part of some units. It attaches to the sink-top control valve and allows a small amount of water to leak out of the tank, if necessary, as the heating water expands.
When you plug the unit into a wall socket, the heating element heats the water. When the water reaches the set temperature, the heating element shuts off. The water temperature is normally set at the factory. But you can usually adjust the setting, following the instructions in the owner's manual.