GSM is a digital cellular technology used worldwide for voice calls and data transfer. It was developed in the 1980s to provide common mobile communication standards across Europe. GSM networks operate using specific frequency bands and are made up of mobile stations, base station subsystems, network switching subsystems, and operation support subsystems. GSM offers services like voice calls, SMS, and data transfer at speeds up to 9.6 kbps. It provides advantages like international roaming, low device/infrastructure costs, and compatibility with other networks.