A telescope mount serves two purposes: it supports the telescope and keeps it steady, and to allows it to swivel around so that you can point it to objects in the night sky. The cost of a good quality mount can often form a large proportion of the overall cost of a telescope. There are two types of mount: the altazimuth mount, and the equatorial mount. Here we discuss the former.
The altazimuth mount is the simpler design and the cheapest to construct. In operation it essentially resembles the mount of a cannon: it allows for up-down motion of the telescope tube (known as the altitude), as well as horizontal motion (known as the azimuth). Many entry level refractor telescopes employ an altazimuth mount, which usually sits on top of a tripod. Dobsonian telescopes are also of the altazimuth type, here the resemblance to a cannon is most striking.
The advantage of the altazimuth mount is its simplicity, ease of use, and cheaper construction (especially true of Dobsonians which tend to be the cheapest telescopes of any given size).
One disadvantage of altazimuth mounts is that they don't have the ability to automatically track objects as they move during the course of the night due to the earth's rotation. In order to keep objects in the field of view one needs to continually adjust the telescope's orientation in both the up-down and left-right directions. (This is less of a problem with equatorial mounts, which are designed to allow for easier tracking by adjusting along just one axis). However, in recent years computerized altazimuth mounts have appeared which use motors to automatically compensate for the sky's motion, though these are generally a great deal more expensive.
When deciding which telescope mount to buy, one important factor to consider is how good the mount is at damping vibrations. The slightest vibration in the telescope apparatus will result in dancing images which are unsuitable for viewing; a good mount should therefore provide a rock solid support for the telescope which minimises vibration.
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