Ok, 3G could mean many things but in the current world it refers to the next generation of data and voice service on cellular networks.
Without getting into gory detail (check http://rickalm.blogspot.com maybe i'll get gory there) it means faster service for downloading / surfing / etc from your mobile. 3G is only usually availible in city settings, so don't be annoyed when you go back to conventional service in the middle of the desert.
