3G or (UMTS) is essentially a high bandwidth mobile telephone network. I can't remember what the actual maximum data rate is, but I think it's around 256 kbps.
It draws heavily from GSM and the later developments GPRS and EDGE (2.5 G), but it differs from GSM in that it is a pure packet network - unlike conventional networks which are circuit switched (during a call, a circuit is reserved for your exclusive use) - although GPRS and EDGE supplement the conventional GSM network with a packet network.
The high bandwidth and packetised nature of the network can allow a number of novel features, including realtime data exchange, walkie-talkie type functionality.
Additionally, there may well be some revolutionary marketing - one new carrier in the UK is currently rumoured to offer unlimited th^H^Hfree voice calls, once they launch their network. We shall see if these are true...