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 Government, who also have the ability of turning on "Selective Availability" where the satellites add a bit of jitter to the signal that they send, so that the GPS is less accurate than normal1? so that the "BadGuys?" can't use GPS to attack the GoodGuys?. Of course this irritates everyone else, so the Europeans are going to put up their own GPS style system.

1?: GPS is pretty inaccurate as it is, because of atmospheric effects and other stuff.