A complete circuit means the electricity is traveling along conductive material like metal from the power source (batteries) through the load (the component that is using the power to light or move) and back to the power source.  It makes a "circle".

So long as the path is unbroken, the electricity will flow and the component will be powered.

A switch is used to easily break  and restore  the path the electricity is traveling, allowing you to turn on and off whatever component is being powered by the circuit. .

If the positive (+) and negative (ground) (-) wires coming from a power source, or anywhere in a circuit that is connected to a power source touch each other, and there is no load to offer resistance, it creates a dangerous situation called a "Short Circuit" as the wires will become very hot, very quickly.  If this ever happens, remove the power source immediately and make sure the wires are no longer touching each other.

Wiring diagram for single LED

Wiring diagram for single LED and switch

Wiring diagram for single LED and button

Wiring Diagram for motor and switch -
change the resistance of the resistor and see what happens to the motor...