GPS stands for Global Positioning System. It is a globally available space-based radio navigation technology which allows any user to determine his/her position anywhere on this planet with a high degree of accuracy. The system owes its existence to the U.S. Government, which has completely funded its development, deployment and maintenance. The technology was originally meant for military use, but was later adopted for the general public.
The system consists of a constellation of 24 satellites, in medium earth orbits (MEO), with their controlling ground stations and the end-user terminals. The satellites use triangulation (more correctly trilateration) technique to pinpoint the location of a user terminal on earth. Ideally three satellites are required to locate a point on the earth's surface, but to take care of timing errors a fourth satellite is required. The controlling ground stations are required to maintain the satellites in their proper orbits and to monitor and correct any changes in their operation. GPS is relatively accurate (around 10 meters). But to attain higher accuracy something called as Differential GPS (DGPS) is used. DGPS uses a stationary point of reference on earth, whose position is accurately known, to locate a GPS terminal with greater accuracy than just GPS.

GPS based Wireless Embedded systems :
One can develop applications on GPS kit which gives you the details about the your location. Applications like Navigation, trekking etc can be built. Most cars to hit the road now come with GPS kits integrated with road-maps to plot routes to user destinations. GPS can also be integrated with GSM, CDMA and other similar technologies in order to get more functionalities like Location Based Services (LBS).