Say you have 120 volts a.c. sending a current thru a light bulb. The bulb will stay on as long as there is voltage. To shut the bulb off we need to break the circuit and stop the current flow. This can be done by placing a switch in the line. A switch is nothing more than a pair of contacts that open and close.
Computers are basically on and off switches.
Industrial motors turn off and on by switches. Just about everything electrical has some sort of switch loop. There are two principal effects of electric current. The heating effect and magnetic effect. Whenever an electric current passes through a material there is a heating effect due to the current.