GSM, which stands for Global System for Mobiles, is a communication technology used in mobile phones in different parts of the world. GSM and CDMA are the two chief mobile phone technologies, and different carriers use these different technologies.
GSM and CDMA communication technologies are not compatible with each other. This means that if a mobile phone is built to use GSM, it cannot connect to the CDMA network and vice versa. Phones that run on the GSM network use small chips called SIM cards to store data. It is easy to swap SIM cards between phones running on the GSM network.