Nikkor means it's a Nikon lens.
AF means it autofocuses on bodies that can do that. With digital SLRs, it means that it will only autofocus on bodies that have a focus motor built in (i.e., not on the D40/D60/D3000/D5000).
28-200mm is the focal length--its zoom in quantitative terms. It's a wide angle to telephoto on full-frame, a widish to very telephoto on a crop body.
f/3.5-5.6 is its maximum aperture(s) at the ends of the zoom range, so at 28mm, the widest the aperture can be set is f/3.5, and at 200mm, the widest it can be set is f/5.6.
G means it's a G-mount lens, and has no aperture ring.
IF means it doesn't change its length as you focus it.
ED means Extra-low Dispersion glass was used in the lens to control chromatic aberration.
Nikon's website has
a glossary where you can look up most of this stuff, and I'd recommend looking at
this lens primer to learn the basics of lenses.