the difference is either according to the technology or the data rates
1g means the wireless comm. systems first used for civilians and used fdma
For 2G the biggest motivation was that FDMA required too much of the very expensive frequency bands and the return wasn't good either - a handful of customers over the network. So, service providers invested in research and development for TDMA based access - more users
The motivation for 3G was to provide users services of wireless connectivity for data services.
Of course, this has not been too successful, UMTS was standardized in 1999 and hasn't been a revolutionary hit by any means.