2G (or 2-G) is short for second-generation cellular technology. Second-generation 2G cellular networks were commercially launched on the GSM standard in Finland by Radiolinja (now part of Elisa Oyj) in 1991.  Three primary benefits of 2G networks over their predecessors were that phone conversations were digitally encrypted; 2G systems were significantly more efficient on the spectrum allowing for far greater wireless penetration levels; and 2G introduced data services for mobile, starting with SMS text messages. 2G technologies enabled the various networks to provide the services such as text messages, picture messages, and MMS (multimedia messages). All text messages sent over 2G are digitally encrypted, allowing for the transfer of data in such a way that only the intended receiver can receive and read it.
After 2G was launched, the previous mobile wireless network systems were retroactively dubbed 1G. While radio signals on 1G networks are analog, radio signals on 2G networks are digital. Both systems use digital signaling to connect the radio towers (which listen to the devices) to the rest of the mobile system.
With General Packet Radio Service (GPRS), 2G offers a theoretical maximum transfer speed of 50 kbit/s (40 kbit/s in practice).  With EDGE (Enhanced Data Rates for GSM Evolution), there is a theoretical maximum transfer speed of 1 Mbit/s (500 kbit/s in practice). 
2G technologies can be divided into time division multiple access (TDMA)-based and code division multiple access (CDMA)-based standards depending on the type of multiplexing used. The main 2G standards are:
- GSM (TDMA-based), originally from Europe but used in most of the world outside North America. Over 60 GSM operators were also using CDMA2000 in the 450 MHz frequency band (CDMA450) by 2010. 
- PDC also known as JDC (Japanese Digital Cellular) (TDMA based), used exclusively in Japan
- iDEN (TDMA-based), proprietary network used by Nextel in the United States and Telus Mobility in Canada
- IS-136 a.k.a. Digital AMPS or D-AMPS (TDMA-based, commonly referred as simply 'TDMA' in the US), was once prevalent in the Americas, but most have migrated to GSM.
2G networks were built mainly for voice services and slow data transmission (defined in IMT-2000 specification documents), but are considered by the general public to be 2.5G or 2.75G services because they are several times slower than present-day 3G service.
2.5G (" second and a half generation ") is used to describe 2G-systems that have implemented a packet-switched domain in addition to the circuit-switched domain. It does not necessarily provide faster service because bundling of timeslots is used for circuit-switched data services (HSCSD) as well.
The first major step in the evolution of GSM networks to 3G occurred with the introduction of General Packet Radio Service (GPRS). CDMA2000 networks similarly evolved through the introduction of 2.5G. Its approach centred on the use of packet data. Up until this time all circuits had been dedicated to a given user in an approach known as circuit switched, i.e. where a complete circuit is switched for a given user. This was inefficient when a channel was only carrying data for a small percentage of the time. The new packet switched approach routed individual packets of data from the transmitter to the receiver allowing the same circuit to be used by different users. This enabled circuits to be used more efficiently and charges to be metered according to the data transferred.
GPRS networks evolved to EDGE networks with the introduction of 8PSK encoding, While the symbol rate remained the same at 270.833 samples per second, each symbol carried three bits instead of one. 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. EDGE was deployed on GSM networks beginning in 2003—initially by AT&T in the United States.
EDGE is standardized by 3GPP as part of the GSM family and it is an upgrade that provides a potential three-fold increase in capacity of GSM/GPRS networks.The 2G digital service provided very useful features, such as caller ID, call forwarding, and short messaging.
Various carriers have made announcements that 2G technology in the United States is in the process of being shut down so that carriers can reclaim those radio bands and re-purpose them for newer technologies (e.g. 4G LTE). AT&T's 2G GSM service was shut down in January 2017.    This shutdown had a notable impact on the electronic security industry, where many 2G GSM radios were in use for alarm signal communication to central station dispatch centers. 2G GSM radios were required to be replaced by newer generation radios to avoid service outages.  Verizon plans to shut down its 2G CDMA 1X network by 31 December 2019.  Whereas, T-Mobile US has postponed shutdown of their 2G network until 2020. 
Asian and Pacific
Reliance Communications, a group led by Reliance ADAG, decided to shut down its entire 2G network at the end of November 2017. It is the first operator in the country to do so. 
T-Mobile Netherlands will shutdown 2G services by 2020. 
Telecommunications in Switzerland is mainly operated by state-owned Swisscom, and the two privately held Salt and Sunrise Communications AG as these companies have a license to operate 2G. Swisscom will cease 2G services due to its "public service requirements" only by 1 January 2021. 
Sunrise Communications AG has announced plans to phase out its GSM network by the end of 2018. GSM, GPRS and EDGE will be ended by the end of 2018 in favour of expanded 4G and 4G+ coverage.