A circuit is a part of a closed loop that electrons travel on. This source of electricity gives electrical energy in the circuit itself. Unless a circuit is complete or making a full circle to the source, no electrons will move. Generally some appliance uses electricity in a circuit, whether by providing light or heat.
In common use, a pre-packaged three wire bundle of copper wires (Romex) starts at the electric panel and is run though the home attaching as many fixtures as is appropriate to the wire set. Electricity flows out the black (hot) wire and back to the panel via the white (neutral) wire. The third, non-coated copper wire is the ground, which is a safety feature designed to attract any loose electricity and allow it to run safely to the ground. A ground wire prevents shock. Some circuits, like household receptecles, will have as many as 8 or 10 devices and some, such as kitchen appliance circuits are dedicated to one device.
Switching a breaker or removing a fuse at the main panel can turn off circuits. One should never work on a device without turning off it’s circuit.
There are two power levels of circuits, 110 and 220 Volts. Most of the devices in a home require less power and are on 110 Volt circuits which have one hot wire. Examples include lights, receptacles, dishwasher, refrigerator, microwave and washing machine. Modern gas appliances also use a 110 circuit for their igniter and safety sensors. Examples include gas ranges, gas cook tops, gas dryers and gas furnaces. Appliances that need to create large amounts of heat are generally 220 Volt devices and need two hot wires to function. Examples include electric furnaces, air conditioning compressors, electric cook top, electric ovens and the cloths dryer.