1G is the first generation cellular 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.
In Telecommunications, 4G is the fourth generation of cellular wireless standards. It is a successor to the 3G and 2G families of standards. In 2008, the ITU-R organization specified the IMT-Advanced (International Mobile Telecommunications Advanced) requirements for 4G standards, setting peak speed requirements for 4G service at 100 Mbit/s for high mobility communication (such as from trains and cars) and 1 Gbit/s for low mobility communication (such as pedestrians and stationary users)
A 4G system is expected to provide a comprehensive and secure all-IP based mobile broadband solution to laptop computer wireless modems, smartphones, and other mobile devices. Facilities such as ultra-broadband Internet access, IP telephony, gaming services, and streamed multimedia may be provided to users.
PRE-4G technologies such as mobile WiMAX and Long term evolution (LTE) have been on the market since 2006 and 2009 respectively, and are often branded as 4G. The current versions of these technologies did not fulfill the original ITU-R requirements of data rates approximately up to 1 Gbit/s for 4G systems. Marketing materials use 4G as a description for LTE and Mobile-WiMAX in their current forms.