"G" stands for "generation" as in 1st, 2nd, 3rd, etc. Each new generation is able to upload and download significantly more data (faster throughput) than the previous generation. This is done two ways. The first is through increasing the size of the portals.

 

The second way is by designing new ways to compress and/or send the digital data by either increased compression of the packets of 1's & 0's or by designing better ways to send them. 

 

Example: There never would have been an iPhone without the 3rd generation technology.

 

If you just cannot get enough information, refer to the detailed explanation of "generation" differences from Farpoint Group's demo at Telecombooksblog.com.