It's called a 3 way circuit. Basically the switches share a common wire so they can both operate the light independently of each other. Usually it's found where you have the need to turn a light on when entering one place and off when exiting in another place.
I'll be wiring at least one in when I get my basement remodel done since there will be a guest room down there. Basically, you turn on the light to go down the stairs and then turn it off when you leave that room to go to the bedroom after you've gotten safely down the stairs.
Rick
