You flip the switch at one entrance and the light goes on, flip another switch at another entrance and the light goes off, pretty cool! A great convenience until you flip a switch and nothing happens. To trouble shoot this issue you need to understand how a 3-way switch works.
Here is a great video on how this switching system works.
Now you know the secret of a 3-way switch, but what happens when you add another switch in between? You then need a 4-way switch to get it to work. Click Here to see how this works.