GSM, GPRS and EDGE (2G)
Updated
GSM is the second-generation (2G) mobile standard, first deployed in 1991; GPRS and EDGE add packet data to it, at tens to a few hundred kbit/s. Its specifications are now maintained by 3GPP.
In practice
Many operators have switched off 2G or have announced a date. A device with a 2G-only module stops working the day its network closes, so check the shutdown dates of each target country before deploying 2G hardware.