Reflecting, Refracting and Cassegrain Telescopes
Before we get into more of the nitty-gritty of telescope buying, there are a few things you need to know to understand what the heck you're looking for. Telescopes are fairly simple instruments, but they're not one-size-fits-all. In fact, before stepping foot in a store -- or clicking through to a link online -- you should probably have some idea of the two most common scopes for astronomers.
There are two main types of telescopes: refractive and reflective. A refractor uses refraction (how convenient!) to bend light to the optical lens. Refraction is when a curved lens is used to bend light as it passes from the atmosphere to the glass of the telescope lens. The curve of the lens -- and the subsequent bending of the light waves -- ensures that when you look in the eyepiece, the image you're seeing is clear, as the light has been faithfully reflected.
A reflective telescope uses mirrors, not glass lenses, to reflect light. (Remember that what our eyes take in is simply a reflection of light, so the telescope is acting as a "larger," or more magnified, retina.) By using a primary mirror in a reflective scope (or an objective lens in a refractor), you're going to bring a lot of light from a distant object nearer, and clarify (or focus) the image. The eyepiece lens in both a refractor or reflector scope is then going to take that large amount of light and magnify it, so that the tiny, clear image is now a larger, clear image. You can get a good quality reflector scope from reputable brands like Orion for around $120 dollars.
So that's all well and good; a refractor telescope bends light to get us an image close-up and clear, while a reflector uses mirrors to project the image to us. Which one is the one that will let you see footprints on the moon for less than $100?
Fortunately, both refractive and reflective telescopes are extremely useful and can be excellent quality. Unfortunately, they also have very real physical limitations. While a refractor will reflect an extremely good contrast because there's no obstruction between the objective lens and the eyepiece, you can also see a chromatic aberration in the image sometimes, meaning the colors appear a bit fuzzy at the edges.
And while refractive scopes offer great images, their lenses are nearly 10 times as expensive as an equivalent reflective mirror [source: Bakich]. That can make the telescope themselves quite a bit more expensive than reflective, in general, but keep in mind that brands like Celestron offer introductory refractory scopes for around $100. Because of the secondary mirror in reflective scopes, there is some loss of contrast, and there can also be a fuzziness around the edges of the field of view.
Now let's scan the horizon for some more concrete things to look for when buying your telescope.