Here are a few links:

http://www.garmin.com/aboutGPS/ -- note the pdf file links at the bottom. They are quite good.

http://www.lowrance.com/Tutorials/GPS/gps_tutorial_01.asp

http://www.trimble.com/gps/

http://www.pbs.org/wgbh/nova/longitude/gps.html

My favorite book on the topic is "GPS Land Navigation" by Michael Ferguson. The section that discusses specific models is too old, but it is very well written and describes use of GPS's, maps, and compasses, both individually and combined.