In household electric circuits the standard wiring used has three individual, insulated wires inside an outer wrap of additional insulation. Two of the individual wires are color-coded: black for the hot line, white for the neutral line, and a third, bare ground wire.
In order for any electric device, such as a toaster, microwave, light, etc. to work it has to be part of a closed circuit. A closed circuit means the electric current flows from a source, through the device using the electricity, and then back to the source. This circuit, or loop, must be closed (complete) in order for the device to work. A switch is an intentional break in a circuit which allows someone to either have current flowing or off in a particular circuit.
In practice, the electric current flows from the main circuit panel through the black or hot line, then into the device. The return line (the neutral) completes the circuit backto the main panel. The third ground wire is there to protect you if there should be a break or defect in the wiring so the current will go to ground instead of through you.
An electric circuit is simply any closed conducting path between the two terminals of a voltage source. Often, the lower voltage terminal is rated at 0V, or equivelant with ground.
In household wiring, the leg of the circuit that provides high voltage is known as the hot, or active, wire (usually colored red or black). The white wire is the neutral wire, and since it has 0 V is known as the neutral wire. However, there is just as much current flowing in the neurtal wire as in the hot wire.
Note that the neutral wire in most modern household circuits is not the same as the ground wire. The grounding wire should not carry any current unless something is wrong; it is in place as a safety mechanism.