A servo is a motor that can run just as effectivly in both clockwise and anticlockwise direction, it has an encoder attached to track its movement.
With a controller card the servo can be driven to a position and hold that position.
Many CNC machines use servos to drive the working tool into a desired position

http://cnczone.com/gallery/data/500/2793servo.gif

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http://www.cnczone.com/forums/showthread.php?t=8011&page=1&pp=10&highlight=servo