Analog cellular phones were the first generation while digital marked the second generation. 3G is generally defined, but generally includes high data speeds, always-on data access, and greater voice scope.
The high data speeds are possibly the most obvious feature, and certainly the most hyped. They enable such advanced features as live, streaming video.
There are several different 3G technology standards. The most prevalent is UMTS, which is based on WCDMA (the terms WCDMA and UMTS are often used interchangeably).