A most basic electrical circuit will have three components
- A source of energy. It could be a battery or an electrical outlet in the wall
- A load. A load does useful work. A load could be a light bulb or a motor
- Conductors. A conductor would be your wires to transfer the electrical energy to your load
While not entirely needed, most circuits will include a switch. A switch will stop the flow of electricity through the conductor to the load.
As you can see, with an open switch, power cannot get past the switch to turn on the lights.
With the switch in the closed position, power is allowed to flow through the circuit and turn the lights on.