Potentiometers typically have three terminals.  One terminal for each end of the resistive track (two in total) and one for the wiper.  The wiper terminal is usually located between the two end terminals.  If you have a forth terminal it would represent a tap in the resistive track.  Taps are usually located at the mid-point or center of the resistive track; i.e. a center-tap.

A tap is used to alter the resistance curve for special applications.  For example a common application for a center tap would be a throttle control for a powered wheel chair.  In the potentiometer center position there would be no movement of the chair.  Moving the potentiometer to one side of the center position would cause the chair to move forward and moving it to the other side would cause it to move backward.  Center taps are also used for balance applications in audio equipment.