There's a couple of major differences between telescopes designed for terrestrial and astronomical viewing. The first concerns how they are mounted on their tripod. 'Regular' (i.e. terrestrial) telescopes can be swivelled horizontally, or vertically - just like a camera on a tripod. This is called an altazimuth mount, or AZ for short. But astronomical telescopes generally use what is called an equatorial mount, or EQ, which has the vertical axis tilted over 23.5 degrees so its parallel with the Earth's axis of rotation. This makes it easier to track stars as they move across the sky, and you can find an object of interest by setting its coordinates (RA & Dec) on the axes' scales.

The other major difference is that usually (not always) the telescope is a reflector, not a refractor. A refractor has a lens (like a camera does), a reflector uses a concave mirror (like a shaving mirror) like Hubble does, to bounce the light back up the tube to a little mirror, which then bounces it out the side (or sometimes, back down the tube through a hole in the mirror) to an eyepiece. These telescopes look 'short'.

A further, minor difference is that images in terrestrial telescopes are 'right way up', in astronomical telescopes they are 'upside down'. However, you can add an 'erector' lens to convert the astronomical image to a terrestrial one.

One more factor to consider, is aperture. This refers to the diameter of the lens or mirror. The larger it is, the more light it will gather - this is especially important for astronomy so that you can see dimmer or more distant objects. Be advised that some cheap children's telescopes (e.g. Oxbridge branding, again) declare the diameter of the lens, but inside the telescope there is a disc with a smaller hole in it, so that the effective aperture is substantially less. They do this because cheap plastic lenses give really bad images except in the central portion - which only give bad images.

So, when buying a telescope, be aware of the main intended use. You can use a terrestrial telescope for astronomy, but serious astronomers will do better with at least an EQ mount - it gets tedious trying to track objects on 2 axes at once!

6 Things To Know About How To Buy a Telescope at About.com Which Telescope is Best for Me? at Starizona.com