While very similar in appearance, motors and servos are suited to distinctly different types of tasks. Servos are incapable of making a full rotation. Vex Servos are limited to about 100 degrees of rotation, but have the invaluable ability to self-correct to maintain any specific position within the set boundaries.
Like motors, Servos operate off of a PWM signal between 0 and 255 (or -128 to 127, depending on development environment), except servos use that signal to move to a specific position at a constant speed set by the manufacturer.