The G in wireless networks refers to the generation of the underlying wireless network technology.
Technically generations are defined as follows:
1G networks (NMT, C-Nets, AMPs, TACS) are considered to be the first analog cellular systems which started early 1980s. There were radio telephone systems even before that.
1G networks were conceived and designed purely for voice calls with almost no consideration of data services, with the possible exception of built-in modems and some headsets.
GSM supports Circuit-Switched Data (CSD) allowing users to place dial-up data calls digitally, so that the network's switching station receives actual ones and zeroes rather than the screech of an analog modem.
2.5G networks (GPRS, CDMA200 1x) are the enhanced versions of 2G networks with theoretical data rates up to about 144 kbit/s.
GPRS offered the first always-on data service.
3G networks (UMTS FDD and TDD, CDMA200 1x EVDO, CDMA200 3x, TD-SCDMA, Arib WCDMA, EDGE, IMT-2000 DECT) are the cellular networks that have data rates of 384kbit/s and more.
The UN's IMT-2000 standard requires stationary speeds of 2Mbps and mobile speeds of 384kbps for a true 3G.
4G networks is the successor of 3G networks.
4G networks have the high data rates which is about 2200 Mbps.
Actually 4G technology is based on the IP services.