GPS stands for 'Global Positioning System' and consists of 24 orbiting satellites, an earth based control system and the GPSR (GPS receiver) that you use. The satellites are owned and controlled by the US Department of Defence. There is no charge for using the system.
Selective Availability (SA), an intentional degradation of the signal, was once used by the military to reduce accuracy. The US government turned off SA in May 2000.
Currently the accuracy is 5-10 metres 95% of the time.
When giving a position 2 types of grids are in common use; the marine and aviation sectors give positions in latitude and longitude with each given in degrees, minutes and decimal minutes. Users of topographical maps use the UTM grid where positions are given in eastings and northings.
Also, the datum is important. This specifies the shape and centre of the earth position that was used to create a map. Positions from different datums can vary by several hundred metres. The default datum for most GPSRs is 'WGS84'. Many countries are in the process of converting their national datums to WGS84 or an equivalent.
GPSRs have many different datums and units that they can be set to to match your paper maps. Just remember to advise which datum you used when exchanging wayspoints and routes.
When selecting a GPSR there are 3 basic types;
1. Basic unit giving a grid based position and basic waypoint, route and tracklog capability
2. Moving map unit with all the basic features but also able to display a map with your position laid on the map. Advanced units may also have turn by turn 'auto-routing' which requires special mapsets.
3. PC or PocketPC mapping display connected to basic GPSR.
This site promotes the use of moving map units as the most useful and robust for motorcycling and other active pastimes such as 4WDing, ATVing, Skidooing etc
When purchasing a moving map GPSR at least half of your decision should be made on the quality of the maps available for your area. This can and does vary with different manufacturers and in different parts of the world.
Most of the information here is about Garmin units because this is the brand that I am most familiar with. If you have info on other brands then your contribution would be appreciated.
Also, you may note that this information has an Australian bias. Again, this is where I am based and other national viewpoints are welcomed particuarly in regard to mapping.