What are 1G, 2G, 3G and 4G networks ?

The "G" in wireless networks refers to the "generation" of the underlying wireless network technology.

1G- This was the first generation of cell phone technology.1G networks are considered to be the first   analog cellular systems.1G networks were purely designed for voice calls only.
                              1G-Analog(Used only for voice calls)

2G-The second generation of cell phone transmission. 2G networks are the first digital cellular systems.It's offering improved sound quality,better security.A few more features were added to the menu such as simple text messaging.
                              2G-Digital,Voice(GSM)
                              2.5G-Added data transmission(GPRS)
                              2.75G-Faster data transmission(EDGE)

3G-The third generation cell phone technology. 3G networks are newer cellular networks which have data rates 2 mbps and more.Web browsing, email, video downloading, picture sharing and other Smartphone technology were introduced in the third generation. Video calling is available in this network.
                             3G-Digital, supported data, packet switched (WCDMA,EvDo)
                             3.5G-Faster downlink data (HSDPA)
                             3.6G-Faster uplink data (HSUPA/HSPA)
                             3.75G-More enhanced HSPA

4G-The 4th generation of mobile phone communication standards.The speed and standards of this wireless technology needs to be at least 100 Megabits per second and up to 1 Gigabit per second to pass as 4G.LTE and WiMAX are parts of this generation.