A clock drive is a system of motors and gears used with an equatorial mount. When observing the sky with a properly aligned mount, it compensates for the Earth's rotation, keeping celestial objects centered in the field of view.
A clock drive is essential to do astrophotography. The motors can be AC or DC.
The gears are usually either worm gears or spur gears.
A GOTO mount also uses a motor drive and additional controlling firmware not only to track, but also to align with stars and find objects anywhere in the sky.