3G refers to third generation. It is the third generation of wireless mobile telecommunications technology. It is the upgrade for 2G and 2.5G GPRS networks. Each generation is characterized by higher data rates and new frequency bands.
High data speed are possibly the most prominent feature, and absolutely the most popular for now. They enable such feature as live streaming video.
There are 3G technology standards. The most prevalent is UMTS, which depends on WCDMA (the terms WCDMA and UMTS are frequently used reciprocally).