What does a GPS actually do?
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With any standard GPS, they will allow you to receive co-ordinates to the accuracy of around 15 metres on average. How GPS work is through satellites. The GPS sends a signal to 24 satellites circling the earth and then using "triangulation", the receiver will tell you where you are. First off though, you will need to have a lock onto these satellites. Most GPS's need at least 3 locked on satellites. Make sure you have a clear view of the sky to be able to obtain connection with the satellites. GPS are great, but trees, cloud cover etc. can affect on you achieving a connection with the satellites, and making you go back to your map and compass.
Garmin have now installed WAAS into most of their GPS systems. WAAS stands for Wide Area Augmentation System, which Garmin claim that GPS will then be able to receive accuracy up to 3 metres. WAAS are basically 3 geostationary satellites and a complex network of ground stations. In the Europe, the equalvent of WAAS will be EGNOS. Every year EGNOS/WAAS will be released by governments after testing, but since 2000, it still has not been released......but all GPS users are hoping that it will finally be available in 2006.
Using "Waypoints", you can create routes on Mapping software such as "Memory Map" and then transfer these to your GPS, so you can then follow a route which can allow the GPS to show distance covered/remaining, predicted" finishing times, average speed etc. These can be helpful in most situations, but unfortunately, the GPS cannot allow time for accent/decent. When routes are followed on the GPS, the GPS will not allow time for accent/decent, which can make the predicted time pretty useless.
You will not always have to use a route to find an area, but if you draw a route using the GPS to an area, the unit will draw a straight line to the designated area you have selected. This in not helpful as usually routes are never straight lines so the distance etc. will be out.
Whether you are following a route or not, the GPS will draw what is called "a track". This is basically a trail of what you have walked with speed and time attached to it. This can then be transferred to your mapping software from your GPS and you can actually look at how far off the path you went! These tracks can be reversed i.e. if weather gets to and and you need to turn back as the fog heads in.
The amount of waypoints, routes and tracks is determined on how much memory your GPS contains. This can be either 8mb,16mb or 24mb.

The best place to buy one is Ebay. I picked up mine for £100 including P&P which is a great price for an item which I use everytime I go walking and for even long distance car journeys to work out how long till we reach the destination!