For any electric appliance to work, an electrical circuit must be present. Without a circuit to link together the individual components that make up an electrical appliance, the components would be merely a collection of disparate bits. What joins all the components together into a system is a circuit.
For a circuit to exist, two conditions must be met. Firstly, there must be a complete and unbroken pathway for the electrical current to flow. The electrical current must be able to flow from the voltage source, through all the various wires, switches and components placed in its path, and back to the voltage source. The circuit is said to have continuity when this condition prevails.
The second condition that must be fulfilled is that that there be a voltage difference between two points of the circuit. This voltage difference is called potential.