In most vehicles, the car headlight switch is three switches in one. A single switch operates one circuit to operate the parking lights, a second for driving lights and a third for high beams. In some cars, the high beams use the normal driving lights but add another light on top of them. In other cars, the high beams are a single discrete system separate from everything else.
Most headlight switches use an electromechanical device called a relay to control the lights. A relay has a small tab of metal with a magnet attached to it and a coil of wire nearby. When an electric current flows through the coil, it creates a magnetic field. This field attracts the magnet, pulling the switch closed. When you turn on your driving lights, a small electric current will flow through the control switch to a relay controlling those lights. It closes that relay, allowing a much larger current to flow from the battery to the headlamps. The high beams and parking lights are controlled by their own relays in exactly the same way.
Some newer cars use light sensors to control the relays. When the sensors detect daylight, the dim parking lights turn on, but the brighter driving lights go off. When the light levels get lower, the cell automatically sends a signal to the driving lights, turning them on. Usually, the driver can manually turn on and off the high beams for extra lighting.
