G means generation.
2G is migration from analog to digita and supports only voice (Those could do data but only as analog modem (using the whole channel) at 9.6Kbps)
Technology: GSM, D-AMPS, PDC
2.5G added dedicated digital data (GPRS)
3G is all digital with the support of data but still circuit switched
Technology: UMTS / WCDMA, EvDO
3.5G faster data, added true always-on / packet data (HSPA) with data rates of 2Mbps of Mbps
4G wireless broadband with Pure data: voice as VoIP
Technology: OFDMA (LTE & WiMAX) with data rate 10 Mbps - 100 Mbps
4.5G term not widely used but some people say that is LTE-Advanced
5G: not yet defined
