A GPS can give you your position on the planet by receiving data from the positioning satellites used by the US military. Now that the civilian signal is not degraded, you should be able to get your exact location within a 50 foot radius anywhere on earth (assuming you're not in a deep canyon or indoors so that your unit can see the satellites).
I got my unit so that I wouldn't get lost trekking solo in Pakistan and India, and also just for fun. I chose the new eTrex available from Garmin (cheaper from REI) due to its small size and cheap price. I must say I'm not that impressed with the unit:
That said, I'm at least having fun with it. Haven't yet used it in a life-saving situation, but it makes it a lot easier to navigate around some of these crazy alleyways in India and Nepal.
Page Revised 8/31/00