3G Stands for 3rd-generation. Analog cellular phones were the first generation. Digital phones marked the second generation (2G).
3G is loosely defined, but generally includes high data speeds, always-on data access, and greater voice capacity (more simultaneous calls per tower.)
The high data speeds are arguably the most important feature, and certainly the most marketed. They enable such advanced features as live, streaming video.
There are several different 3G technology standards. The most prevalent worldwide is WCDMA (also known as UMTS.) WCDMA is the 3G technology of choice for most carriers that used GSM as their 2G technology.
The other major 3G standard in the U.S. is CDMA 1x, which is an evolution of CDMA 2G technology. There are several types of CDMA 1x, each offering different data rates and levels of compatibility with 2G CDMA. CDMA 1xEV-DO Rev A. became the most common.
3G networks are being augmented - and eventually replaced - by 4G networks, which offer faster data speeds using more advanced radio technology.