USSD is a mobile technology used to offer a menu of services for the user to chose from. It is mainly used to offer mobile banking services, mobile surveys and other services that require to give the user a guided choice.
It usually consists of number that start with * and end with #. It is an interactive sms session with a client. A customer dials a certain code eg. *100# to access a certain service or array of services. On dialing the code, the customer is given a menu of choices from which he picks on to a logical conclusion.
The service is either paid for by the customer or the organization.