GSM is abbreviation of Gobal System for Mobile is supportive technology network 2G. At this time GSM is network technology that used by most cellphones around the world.
GPRS or network 2,5 G is abbreviation of General Packet Radio System is technology above GSM that can break data into packages look like the working of internet.
Whereas EDGE is abbreviation of Enhanced Data GSM Environment is technology above GPRS ably quicker.