Three Way Switch
The three way switch is always used in pairs and allows you to turn a light or receptacle on and off from two different locations. These switches have no "on" or "off" markings because the on and off positions will vary as the switches are used.
The three way switch has three terminal screws and here's where it can start getting confusing for most people (but it won't be for you, don't worry). The black "hot" wire is attached to the darkest screw terminal marked "COM" for "common." The other two terminals are used to wire up what are called "travelers" or leads for the switch. It makes no real difference which traveler wire goes to what traveler terminal on the switch. The switch will also come with a ground terminal (green screw).
The trick in replacing a old switch is to mark the COM or common wire off the old three way switch before you replace the old switch.
Please note, you cannot use any other type of switch in a three way application, and again, these switches must be used in pairs.