2.5G
2.5G is an ad-hoc stepping stone between 2G and 3G wireless technologies. The term "second and a half generation" is used to describe systems that provide faster services than 2G, but not quite as fast or advanced as newer 3G systems. 2.5G provides some of the benefits of 3G (e.g. it is packet-switched) and can use some of the existing 2G infrastructure in GSM and CDMA networks. Common 2.5G protocols include GPRS. Some protocols, such as EDGE for GSM and CDMA2000 1x-RTT for CDMA, officially qualify as "3G" services (because they have a data rate of above 144kbps), but are considered by most to be 2.5G services because they are several times slower than "true" 3G services.
2G is the current generation of mobile phones. It transmits sounds primarily, in digital form, over a circuit-switched network. Data service is possible, but only via circuit switching.
3G was supposed to be the next (third) generation of mobile phones. It is packet-switched (more efficient, faster) rather than circuit-switched (less efficient, slower), and can transmit data (e.g. pictures, movies, etc...) as well as voice. But, by and large, it requires a different infrastructure than 2G. Due to cost and complexity, rollout of 3G has been somewhat slower than anticipated.
