Acronym for Global Positioning System.
GPS is a set of satellites that know precisely where they are, and the exact time and broadcast this fact. You can then use a GPS reciever which listens to these signals, and uses them to find out exactly where it is.
GPS is run by the American Department of Defence, who used to run a thing called "Selective Availability" where the satellites add a bit of jitter to the signal that they send, so that public GPS recievers were always off by 10-20 meters. This was so the bad guys can't use GPS to attack the good guys. This irritated everyone else, so the Europeans are going to put up their own GPS style system.
At this time (1999-2000) American Army GPS recievers were accurate to about 5cm