USSD (Unstructured Supplementary Service Data) is a Global System for Mobile(GSM) communication technology that is used to send text between a mobile phone and an application program in the network.
Applications may include
prepaid roaming or mobile chatting or using value added services.
USSD is commonly used by prepaid GSM cellular phones to query the available balance. 
It is secured and makes a short real time login session with server of service provider to make  transactions.The connection remains open, allowing a two-way exchange of a sequence of data. 
USSD is more responsive than SMS
USSD messages are up to 182 alphanumeric characters long.
A typical USSD message starts with an asterisk (*) followed by digits that comprise commands or data. Groups of digits may be separated by additional asterisks. The message is terminated with a number sign (#)

National Payment Corporation of India is giving *99# as the USSD code for citizen , enabling  rural India to make electronic banking without internet. It works with all GSM Operators in India.