3G stands for third generation wireless communication technology. Our current wireless communication technology evolves from 1G, 2G, to 2.5G(GPRS). To put it simple, what all these mean is the speed of transmitting data via wireless technologies. 1G transmits data in the lowest rate--it only allows us to "talk", 2G technology improves the data rate vastly, and hence allows us to not only speak, but sms(up to 160 characters) as well, 3G does that even faster(up to 2Mbps in ideal situation), and hence the transmission of pictures, checking of emails, etc are possible.
Our 2G technology can be further classified in to two totally different systems, GSM (TDMA) and CDMA. The evolution of 3G from 2G hence can be accomplished in two paths, via GSM to GPRS(2.5G) to EDGE to WCDMA(which satisfies our 3G requirement) or directly from CDMA.