Turn a switch on and it completes the circuit, letting electricity flow through it. Turn it off and the circuit is broken; the switch creates a gap that stops the flow.
The most common household switch, a single-pole, has two terminals and simply turns power on or off.
A three-way switch has three terminals; a four-way has four. These control a light from two or three locations, such as in a stairwell, at either end of a hallway, or in a large room with more than one entrance.
A dimmer switch controls a light's intensity. Usually you can replace any single-pole switch with a dimmer. However, for a fan or fluorescent light, buy a special switch rated to control those devices.
In addition to the familiar toggle and rotary switches, specialty switches can do everything from turning on when you walk into a room to varying the speed of whole-house fans. Other special-duty switches can be time-programmed or let you know whether a remote light is on or off. Decorative switches include styles that rock, turn, or slide rather than toggle.