Location Tracking & Electronic Monitoring System in simple terms
The GPS tracking and communications device includes a GPS chip which can receive signals from usually up to 12 (of 24) GPS satellites. The more signals it receives the higher the probability to compute its correct location is. The GPS tracking and communications device is ususally capable of storing its locations and report its current location or the locations it has been on-demand via SMS or TCP/IP over GPRS.
The server simply relays any communications from the GPS tracking and communications devices to the client applications and vice-versa over the Internet. Depending on its architecture the server can store data too.
The client application is basically a user interface which facilitates positions requests and displays any incoming position on a digital map. It can also facilitate - in some cases - the generation of specific reports such as tracking history.