An alt-azimuth mount moves up and down and side to side, the way a camera tripod or a human head does. It is the oldest arrangement, the cheapest to build well, and for visual observing it is very often the best one available at the price.
Why it suits looking
Nothing to align, nothing to balance with counterweights, and the eyepiece stays in sensible positions as the telescope moves. You point it at what you want and look. For an observer whose session is an hour on a weeknight, the absence of a setup routine is worth more than any amount of tracking accuracy.
Why it struggles with photography
As an object crosses the sky it also appears to rotate, and an alt-azimuth mount does not rotate with it. Over a long exposure the stars at the edge of the frame trail in arcs around the centre. Short exposures are unaffected, which is why alt-azimuth mounts work well for lunar and planetary imaging and poorly for deep sky work.
The dobsonian, the best version of the idea
A dobsonian base is an alt-azimuth mount made of plywood, and it is steadier than most metal tripods because it is heavy, low and has large smooth bearings. That is why the design gives more aperture per pound than anything else: it spends nothing on the mount and loses nothing in steadiness.
Driven alt-azimuth mounts
Add two motors and a computer and an alt-azimuth mount tracks perfectly well for visual use, which is what almost every modern computerised telescope does. The field rotation is still there and still limits long exposures, and nobody notices it at the eyepiece because the eye integrates nothing over time.
Questions people ask about alt az mount
What is an alt-azimuth mount?
One that moves in altitude and azimuth, up and across, like a camera tripod head. The simplest and cheapest arrangement available.
Is an alt-azimuth mount good enough?
For visual observing, yes, and it is what most observers use. For long exposure photography, no, because of field rotation.
What is field rotation?
The apparent turning of the star field as an object crosses the sky. An equatorial mount cancels it; an alt-azimuth one does not, so long exposures show curved trails at the frame edges.