GSM (Global System for Mobile Communications) is a cellular network standard used by mobile devices. It was developed in the 1980s to establish a common standard for Europe that allowed users to use a single mobile phone across countries. Key components of GSM include the mobile station (mobile device), base station system including base transceiver stations and controllers, and the network switching system including the mobile switching center. GSM uses TDMA and FDMA and has expanded to over 212 countries with over 3 billion subscribers. It provides features like SIM cards, security including PIN codes, and supports voice calls and SMS messaging.