In domestic electrical connections, we have three different types of wiring: live or hot wire, neutral wire and earth wire. Each electrical circuit is composed of these wires, each of which is color coded. The live or hot wire is black in color, while the neutral is white and the earth wire is typically green in color. For an electrical appliance to work, the circuit must be complete. This simply means that the current should flow from a source, through the appliance and back to source. The live wire carries the current to the appliance, while the neutral wire carries it back. The earth wire is for our protection, in case the live wire makes a contact with metal casing of an appliance. When this happens, the current will pass to the earth instead of our body, thus saving us.
A switch is used to connect or disconnect (break) a circuit. When a switch is in 'on' position, the current will flow and when it is in "off" position, the current flow ceases.
Hope this helps.