GSM stands for Global System for Mobile Communications. It is the standard that has been widely adopted worldwide for mobile cellular networks, making it possible for mobile users to make voice calls, send SMS messages, and access the internet through data services. GSM operates on different frequency bands and uses the circuit-switching model. CDMA, while a rival technology, does not operate on the principle of a 'global system' and is not as widely adopted internationally. LTE and 5G are later evolutions of cellular technology standards that are designed to provide higher speeds and improved network efficiency over GSM.