Generation of Wireless Telephone Technology:

First-generation wireless telephone technology or 1G:
It uses analog telecommunications standards. It uses digital signaling to connect the radio towers (which listen to the handsets) to the rest of the telephone system, the voice itself during a call is modulated to higher frequency, typically 150 MHz and up in 1G. 

The main difference between two succeeding mobile telephone systems, 1G and 2G, is that the radio signals that 1G networks use are analog, while 2G networks are digital.

Second-generation wireless telephone technology or 2G: 2G introduced data services for mobile, starting with SMS text messages.
2G technologies can be divided into TDMA-based and CDMA-based standards depending on the type of multiplexing used.

2.5G (GPRS):
GPRS could provide data rates from 56 kbit/s up to 115 kbit/s. It can be used for services such as Wireless Application Protocol (WAP) access, Multimedia Messaging Service (MMS), and for Internet communication services such as email and World Wide Web access. GPRS data transfer is typically charged per megabyte of traffic transferred, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user actually is utilizing the capacity or is in an idle state.

2.75G (EDGE):
GPRS networks evolved to EDGE networks with the introduction of 8PSK encoding. Enhanced Data rates for GSM Evolution (EDGE), Enhanced GPRS (EGPRS), or IMT Single Carrier (IMT-SC) is a backward-compatible digital mobile phone technology that allows improved data transmission rates, as an extension on top of standard GSM.

3rd Generation wireless telephone technology or 3G:
The main technological difference that distinguishes 3G technology from 2G technology is the use of packet-switching rather than circuit-switching for data transmission.
3G allows simultaneous use of speech and data services and higher data rates. Today's 3G systems can in practice offer up to 14.0 Mbit/s on the downlink and 5.8 Mbit/s on the uplink.
By the help of 3G now a mobile user can have acess to these applications like Mobile TV, Video on demand, Video conferencing, Tele-medicine and Location-based services.

Fourth generation of cellular wireless standards or 4G:
A 4G system is expected to provide a comprehensive and secure all-IP based solution where facilities such as IP telephony, ultra-broadband Internet access, gaming services and streamed multimedia may be provided to users.