Every basic wiring system has two types of wires, a hot wire and a neutral wire.  These are represented in a an electrical receptacle (socket) by the two prongs (Figure 1).  The circular prong at the bottom is used in most newer instrument plugs is used for grounding, to prevent possible shocks.

Figure 1

The following diagram (Figure 2) is an example of the simplest wiring diagram possible, a light hooked directly to a socket.  The light is labeled by an 'L'.

Figure 2

The next diagram (Figure 3) shows a switch (rectangle) that operates the light.  When the switch is turned on, the conducting line in the switch touches the other metal contact (green circle) and closes, or completes, the circuit.  The yellow return wire then carries the current to the light.  The neutral wire must still touch the light.

Figure 3

by Pradeep