Great help on a 3-Way wiring diagram from your website. Much appreciated, as I have had the same 3 Way switch, non-operational.
This diagram illustrates a very simple circuit. 
The battery is represented by 4 lines (longer lines to be positive and negative is shorter). Starting from the negative end of the battery, the electrons “circle” through one wire, through the bulb, the other through the wire and then back to the battery so that the complete circuit.
but there is 2 weakness of this circuit 1) light is always on top and 2) constant power is used. How can we turn on the light bulb ‘off’? Well, we can open the lid of the bulb socket, but in the real world is very uncomfortable. (Light bulbs are in gear, on the ceiling and so on). Maybe we can disconnect the power at its source. It is also very comfortable. You’ll have to go into your basement to shut off power setting. Or – much more dangerous – you will have to decide power supply wire before reaching the light socket.
Is there a safe way to disrupt the flow of electrons without touching the physical wires? Sure. This is called a SWITCH!
The interior of a typical household wall switch has a metal strip (B), making contact with the point “A”, thereby completing the electrical circuit to make the light. This clearly would be ‘ON’ position. When isolated lever was moved to ‘OFF’ position, the metal strip was pushed away from point ‘A’, breaking the circuit and the light turned ‘OFF’.
Type of switch (having a lever which “flipped” it on and off) is called a toggle switch.
The common terminal is the middle terminal in the SPDT Knife Switch or if you are using a household switch, it would be the brass colored terminal. 
(the other 2 would be silver colored). This circuit clearly demonstrates what happens when the SPDT switch is moved back and forth. 
Light A goes on and B goes off, B goes on and A goes off and so forth. You can see that this popular switch would have many practical applications: the transmit/receive button on a “2-way” radio, the “high/low beam” switch for your car headlights, the pulse/tone dialing switch on your telephone, and so on.
If you are using the SPDT knife switch, you have a “center off” position, which an ordinary wall switch would NOT have in which case you will need to add an SPST switch for shutting this circuit off. (In electronics work, many SPDT switches have a middle position in which the electricity is turned off to BOTH circuits.
