USSD (Unstructured Supplementary Service Data) is a protocol used by GSM cellular networks to communicate with service providers. It allows for menu-based, two-way communication between a mobile device and application servers via short text messages of up to 182 characters. USSD sessions remain open, allowing for faster interactions than SMS. Some advantages of USSD include not requiring specific hardware or software, providing more user interactions through menus, and allowing services to work while roaming without additional charges. Examples of USSD services include balance checks, recharges, surveys, and social media access.