Andy L , Jan 24, 2010; 04:13 p.m.
So there are a few variables with lenses:
-How "long" is it? The lower the number in mm's, the wider-angle the lens; the larger the number, the more more telephoto it is. On a consumer DSLR, for example a Nikon DX type camera (as your questions seem to imply you are looking at) a 35mm lens is "normal", 50mm is a bit long - you take a few steps back before taking a photo of a person. 85mm is longer than that, and many people consider it a great length for taking portrait shots of people.
-How "fast" is it? The smaller the f/number, the wider the aperture can open. Wider apertures allow more light in, letting you shoot in low light, with a faster shutter, lower ISO, etc., and also blur the background more which is good for some types of shots. Faster lenses are usually more expensive, and may be made with better parts as they are usually higher grade lenses - which is why the 50/1.4D is more expensive than the 50/1.8D.
-How "well made" is it? Higher end lenses usually include more metal and less plastic, and are made with higher grades of glass. This makes them longer lasting and improves the image quality in certain ways.
-Does it have any "special" features? An AF-S lens is usually more expensive than a similar D lens, because an AF-S lens has a built in motor. (Lower end Nikon DSLRs will not autofocus with a lens that is not AF-S because these cameras don't have built in motors.) VR also adds to the cost.
The 18-55 VR lens is not fast and is not as well made as higher end lenses (it's not bad - it's just not as good) but the image quality isn't bad, it has AF-S and VR and it's inexpensive, so it's not a bad starter lens.
Beginners can use 18-200 lenses. The Nikon 18-200 is versatile in that it cover a very large focal range (so if you want only one lens, it can do a lot), has AF-S and VR and is fairly well made, but is not fast.
BTW, the singular of "lenses" is "lens".