I'll give this a shot.
Scope selection depends on your intended target, conditions and gun.
By intended target, when you speak of varmint hunting your talking medium to long range, small targets and typically high power scopes to find and shoot these small critters. If you're talking about just shooting targets at 100 yards, then a high fixed power scope would be more appropriate. For general plinking, a middle of the road variable power scope would be all that you needed.
By conditions, where do you live? Will you be hunting wide open areas or dense woods?
And the gun....what gun are you trying to mount a scope on? What purpose does the gun serve? Once you can answer these questions, you'll have an idea of the type of scope you need.
There are several numbers that describe a scope. For example, 4x36. The first number is the magnification and the second is the size of the objective lens in millimeters (the lens at the opposite end you look through). Also, a 3-9x40 label dictates that the scope has variable magnification from 3 to 9 power with a objective lens of 40mm. A 8-32x50 has a magnification range of 8-32 power with a objective lens of 50mm. The larger the objective lens, the more light the scope lets in and the crisper and brighter the image is. There are some drawbacks to having a large scope though, portability for one. If this is a hunting gun, a large scope is more of a pain than anything else. Also, there are generally two different sizes of main tubes: the American 1" and the 30mm.
Most people don't spend the required amount of money on optics. They get a very nice rifle and put a crappy Tasco on it. A general rule is to spend 2/3 the cost of the gun on optics.
Generally, most guns are over scoped as far as magnification goes. Yes, I'll freely admit, I do it too. The classic hunting scope is a 3-9x40 of quality manufacture. Out here in open California where I do a lot of varmint hunting, I prefer a 4-14 at least. Varmint hunters typically use a higher powered scope for the reasons above, anywhere from 6-18, 6.5-20 and 8-32 magnification.
The standard of optical quality is generally set by Leupold. Some other quality manufacturers are Burris, Bushnell (the Elite series), Kahles, Nikon and Weaver. If you really want to go high-end, look at Schmidt & Bender, Swarovski, US Optics and Zeiss.
Also, take a look at the hunting forum and get an idea of what people use on their guns.
Riflescopes.com is a nice place to look things up and check out the different products.
Good Luck,
Ed