There are at least 4, 5 different ways for you to find out where someone (or a device) is currently located:
Satellite based GPS
This is the free, radio signal based one. Satellites up our heads beeping signals in a constant frequency that can be picked up by GPS (radio) receivers.
You need a device which actually "listens" to the GPS radio signals to get it. They need to carry chipsets providing a combination of RF components and software to correlate and process the location data into latitude, longitude and altitude plus time values.
Phone Carrier based Assistance to GPS signaling
With E911 requirements, carriers were obligated to provide progressively more accurate location information in emergencies to cell phone users.
There are basically two main ways to broadcast digital signals to multiple cell phone receivers: GSM and CDMA.
CDMA radio signals as those broadcasted by the GPS satellites, have a time-stamp signature. This is the key data for trilaterating (also referred to as triangulation) three or more points for determining location of a given receiver.
Assisted GPS combines triangulation results from a cell phone obtained from the time a signal takes to reach it from the cell towers; to the GPS data of known locations.
Being a time-based network CDMA allows for more precise determination of a cell phone and its user. GSM provides a much lower precision and there are attempts to improve its resolution through methods like Enhanced GPS for example.
Cell Tower ID databases
Another approach is to create and refer to a database of cell towers id's to obtain its corresponding location (latitude/longitude). For that customers knowingly (or unknowingly) provide the data to seed a database.
Google Maps for Mobile uses this approach with its My Location feature.
Wi-Fi MAC addresses databases
In heavily populated areas, the use of data from wireless access points associated to their corresponding location extends the Cell Tower database approach.
(Static) IP address can also be used for location.
And there are also proposed standards and implementations of GPS data transport protocols which opens lots of possibilities.
What do you got?
So based on this you have GPS or more precisely Location data depending on how manufactures, developers and phone companies decide what is available for you as customer.
In cell phones some sort of E911 will be available. This can be implemented in several forms. For example, CDMA carriers use a Position Determination Entity, or PDE Server that keeps track of devices location.
Privacy concerns should abound here and in any other case where private data is kept.