A G is a generation. Within each of those generations there are various competingh standards, and various "half step" improvements.
There is also a lot of marketing spin.
1G = analog
Examples include NMT, AMPS, TACS, etc
2G = digital, voice
Examples include GSM, D-AMPS, PDC
Some people claim IS-95 original cdma was 3G: I'd say that as a sophisticated 2G
2.5G added data (GPRS)
2.75G faster data (EDGE)
3G = digital, supported data, but still circuit switched
3.5G faster data, added packet data onto CS architecture (HSPA)
The USA and few ither places had a rival technology ("CDMA"). That was marketed as 3G when it was launched, but initially was probably closer to 2.5G and then upgraded to 3G with the EvD) upgraded.
4G = wireless broadband
OFDMA, flat architecture, true packet switched
Most people say this is LTE & WiMAX, though some people are waiting for an upgrade to LTE-A (based on a rather silly data rate definition).