A three way switch can control a light or set of lights from two different locations. Plus, you can have as many four (4) way switches as you want as long as they are in between two three way switches. It takes 3 wires for a three way switch to work. One common wire and two traveler wires. Both three way switches will have one common and two travelers for each one.
The two common wires will be the black hot feed on one end and the load, which is the black hot wire from the switch to the light fixture, on the other end. The common lug on the three way switch will either make contact with one or the other traveler wires. If the switch is up it connects the common to traveler A and if it is switched down it makes contact from the common to traveler B.
The constant hot feed is hooked to the common lug on one end so it sends the hot to the other switch on one or the other traveler wires. When the hot feed reaches the other three way switch depending on which traveler wire is making contact with that switches common lug, it will either send the power up to the light or keep it at the switch.
Remember one traveler wire will always be making contact with the common. If the right traveler that is carrying the hot feed matches up with the common lug going up to the light, then the light will come on and if not the light will turn off. At either end of the three way switches when you flip the switch it changes the traveler wire that is making contact with the common and if the light is on it will go off and vice versa. Simple stuff huh? If you are having problems with your three way switches below are some links to help you get it figured out.
How To Troubleshoot 3 way switches
Learn The Different Ways To Wire Three Way Switches
