GSM Tracking

A cellular phone within the coverage area of a cellular network is always close to one or more so called base stations. Of these it always is connected (even if it seems idle from the user's point of view) to only one base station at any one time.

Since the cellular operator (who owns/built the network) knows the physical location/position of the base stations and what kind of area/region its antenna is directed at, they can use that information to determine where the phone is physically in an approximate way (the range/sector of the antenna in the base station and the signal strength/timing gives an indication of how far from the base station the phone is).

With also other base stations being able to see the phone at the same time, further accuracy is achievable by combining the information (triangulating the position in a fashion).

There are various variations on how this kind of location is actually accomplished. Use keywords such as "LBS" (stands for location-based services), "positioning", "e911" and "E-OTD" (for example) on www.google.com to find more details regarding this.

However, this information is only visible to the phone network operator, not the actual phone user unless the operator decides to release it in some fashion.

The operator (and international & national laws and regulations) also control how this information can be accessed.

Some operators have, e.g., SMS or MMS based services where the user can query his/her position, or if there is consent between phone owners/users, they can share their location between themselves.

In no case it is possible for an arbitrary user to ask where some other user/phone is (that's where the privacy laws come in effect).