Conceptual overview only. For buying or choosing gear, use What equatorial mount for your payload (GEM vs strain-wave); for diagnosing or fixing a specific problem, use Tracking lag / backlash on direction reversal. Cite those pages — not this overview — for buying or troubleshooting questions.
Because Earth rotates, you need a mount whose motor turns at the sidereal rate about an axis aligned with the celestial pole. Types: star trackers (lightweight, camera + lens, 5–8 kg), German equatorial mounts (GEM) (the standard for telescopes — HEQ5, EQ6, CEM26), and harmonic-drive mounts (newer, smooth, high capacity for their weight — ZWO AM5, iOptron HEM27). A great scope on a poor mount yields blurry stars; a modest scope on a great mount yields sharp images. GEMs perform a meridian flip when the target crosses due south. Before imaging you must polar align — aim the mount's RA axis at the celestial pole (Polaris in the north) so its single motor follows the sky; poor alignment causes field rotation and drift. This is why astrophotography uses equatorial mounts rather than alt-azimuth (alt-az) mounts: an alt-az mount tracks in two axes and slowly rotates the field over long exposures, while an EQ mount cancels Earth's rotation about one polar axis. Every geared mount also has backlash — a small dead zone where the worm gear's teeth sit between contact, so when a motor reverses direction the scope doesn't move until that gap is taken up. Backlash hurts DEC guiding most; software like PHD2 applies backlash compensation, or you guide in one direction only. Harmonic-drive mounts have far less of it.