Difference Between 1G, 2G, 2.5G, 3G, Pre-4G and 4G:
1G is the first generation celullar network that existed in 1980s. It transfer data (only voice) in analog wave, it has limitation because there are no encryption, the sound quality is poor and the speed of transfer is only at 9.6kbps.
2G is the second one, improved by introducing the concept of digital modulation, which means converting the voice(only) into digital code(in your phone) and then into analog signals(imagine that it flys in the air). Being digital, they overcame some of the limitations of 1G, such as it omits the radio power from handsets making life more healthier, and it has enhanced privacy.
2.5G is a transition of 2G and 3G. In 2.5G, the most popular services like SMS (short messaging service), GPRS, EDGE, High Speed Circuit switched data, and more had been introduced.
3G is the current generation of mobile telecommunication standards. It allows simultaneous use of speech and data services and offers data rates of up to 2 Mbps, which provide servcies like video calls, mobile TV, mobile Internet and downloading. There are a bunch of technologies that fall under 3G, like WCDMA, EV-DO, and HSPA and others.