The "G" in wireless networks refers to the "Generation" of the underlying wireless network technology.
Technically generations are defined as follows:
1G network (NMT, C-Nets, AMPS, TACS) are considered to be first analog cellular systems, which started early 1980s. There were radio telephone systems even before that 1G networks were convinced and designed purely for voice calls with almost no consideration of data services (with the possible exception of built in modems in some handsets).
2G networks (GSM, CDMAOne, D-AMPS) are the first digital cellular system launched early 1990s, Offering improved sound quality, better security and higher total capacity. GSM supports circuit-switched data (CSD), Allowing users to place dial-up data calls digitally, so that the network's switching station receives actual ones and zeroes rather than the screech of analog modem.

2.5G networks (GPRS, CDMA2000 1x) are the enhanced versions of 2G networks with theoritical data rates upto about 144 Kbit/s. GPRS offered the first always-on data service.
3G networks (UMTSFDD and TDD, CDMA2000 1x EVDO, CDMA 2000 3x, TD-SCDMA, Arib WCDMA, EDGE, IMT-2000 DECT) are newer cellular networks that have data rates of 384 Kb/s and more.
The UN's IMT-2000 standard requires stationary speeds of 2Mb/s and mobile speeds of 384 Kb/s for a "true" 3G.
4G networks 4G is all about faster Internet speeds. It offers no improvement in making calls or sending texts but very fast web-experience compared to 3G. Fourth generation (4G) technology offers many advancements to the wireless market such as download-link data rates well over 100 Mb/s, low latency, very efficient spectrum use and low cost implementations.

4G is all about convergence of wired and wireless networks, wireless technologies including GSM, wireless LAN, and Bluetooth as well as computers, consumer electronics, communication technology and several others. It is also referred to by "MAGIC" which stands for mobile multimedia, Anywhere, Global mobility solution over, Integrated wireless and customized services.
It is an all IP-based integrated system will be capable to provide 100 Mb/s for high mobility and one Gb/s for low mobility, With end to end QoS and high security, And will offer VIRUS services at any time as per user requirements, anywhere with shemless interoperability, at affordable cost. The user services include IP telephony, ultra broadband internet access, Gaming services and high definition television (HD TV) streamed multimedia underneath 4G will use technologies like WiMAX and LTE (Long Term Evolution) etc.

5G networks 5G is generally seen as the fifth generation cellular network technology that provides broadband access. 5G NR speed in sub-6 GHz bands can be slightly higher than the 4G with a similar amount of spectrum and antennas, though some 3GPP 5G networks will be slower than some advanced 4G networks, Such as T-Mobile's LTE/LAA network, Which achieves 500+ Mb/s in Manhattan.
it is still in devlopement.