By definition, an outlet is any device that allows access to electricity through a device. This includes luminaries for lighting and receptacles for appliances. Some luminaries have threaded sockets for light bulbs that make contact with two or three terminals. Other light bulbs fit into ceramic or plastic slots and make contact with the outlet terminals. Receptacles accept plugs attached to cords, which carry the electricity from the outlet to a device such as a vacuum cleaner.
Common incandescent light bulbs screw into a socket. The wall of the socket is a metal terminal and at the bottom is a second and sometimes a third terminal that makes contact with the bottom terminals of the light bulb. When power is applied to the outlet, it flows through one terminal, through the light bulb filament and out the other terminal. In the case of a three-way bulb, electricity can flow through either or both of two terminals. If it flows through one terminal, the bulb emits a lower amount of light. If it flows through a second terminal, more light is produced. A third setting selects both terminals and the bulb emits the maximum amount of light.
Common tube lamps come in many sizes from a few inches to 8 feet in length. Some require a different voltage than standard house current and use transformers to convert it. The lamps have terminals at both ends, which are inserted into slots in the outlet. The terminals on the lamps make contact with terminals in the outlet. Common types of slotted luminaries are halogen spotlights and fluorescent lights. Current flows through one terminal, through the lamp and out through the opposite terminal.
Receptacles supply electricity through two or more terminals. When an appliance plug is inserted into the receptacle, the plug terminals make contact with the receptacle terminals, allowing electricity to flow through the plug, into the appliance and back out through the plug and receptacle. In the U.S., there are two common voltages, 120 volts and 240 volts. Two terminals can supply either voltage. Modern 120 volt and 240 volt receptacles are required to include a third ground terminal by the NEC . In some configurations, the 240 volt receptacle also supplies 120 volt, in which case a fourth slot will be present. A 120 volt receptacle allows electricity to flow through a "Hot" terminal, through the device, and returns the current through a second terminal. Receptacles for 240 volts work differently. Two terminals are "Hot" and carry current to the device. Each hot terminal also acts as the return for other hot terminal. If the receptacle also supplies 120 volts, a third terminal called the neutral acts to return the 120 volt portion of the current. The shape of the receptacle slots is an indication of the voltage and current the receptacle is capable of supplying. See the illustration for various receptacle types.