The Global System for Mobile communications (GSM), formerly known as "Group Special Mobile", is a world-wide standard for digital wireless mobile phones. The standard was originated by CEPT and further developed by ETSI as a standard for European mobile phones, with the intention of developing a standard for adoption world-wide.
The standard is open, non-propietary and still evolving. It has obtained wide support especially in Europe, where it is the major standard.
GSM differs from its predecessors most significantly in that both signalling and speech channels are digital. It has also been designed for a moderate level of security.
Yeah, ok, it's a start. Please, add some more
GSM exists in three main versions, based on the band used: GSM-900, GSM-1800 and GSM-1900. GSM-900 (900 Mhz) and GSM-1800 (1.8 Ghz) are used in most of the world, excluding the United States (and Canada?). The United States uses GSM-1900 (1.9 Ghz) instead, since in the U.S. the 900 and 1800 bands were already allocated.
In Europe and other areas outside North America the GSM system initially used a frequency of 900MHz, shortly afterwards the PCN network used the 1800MHZ frequency, nowadays the PCN networks are considered part of the GSM system and many phones are dual-band operating on 900/1800MHz.