USSD :-
Unstructured Supplementary Service Data (USSD) is a protocol used by GSM cellular telephones to communicate with the service provider's computers. USSD can be used for WAP browsing, prepaid callback service, mobile-money services, location-based content services, menu-based information services, and as part of configuring the phone on the network.
USSD messages are up to 182 alphanumeric characters in length. Unlike Short Message Service (SMS) messages, USSD messages create a real-time connection during a USSD session. The connection remains open, allowing a two-way exchange of a sequence of data. This makes USSD more responsive than services that use SMS.
When a user sends a message such as (*123#) to the phone company network, it is received by a computer dedicated to USSD and the computer's answer is sent back to the phone, generally in a very basic presentation format that can easily be seen on the phone screen. The messages sent over USSD are not defined by any standardization body, so each network operator can implement whatever it finds suitable for its customers. USSD can be used to provide independent calling services such as a callback service (e.g., cheaper phone charges while roaming), enhance mobile marketing capabilities, or interactive data service (e.g., stock quotes, sports results).

USSD is commonly used by prepaid GSM cellular phones to query the available balance. The vendor's "check balance" application hides the details of the USSD protocol from the user. On some pay as you go networks, once a user performs an action that costs money, they see a USSD message with their new balance. USSD can also sometimes be used to refill the balance on the user's SIM card and to deliver One Time Passwords or PIN codes.
Some operators use USSD to provide access to real-time updates from social-networking websites like Facebook and Twitter. Wikipedia uses USSD to send articles to some feature phones.
USSD is sometimes used in conjunction with SMS: the user sends a request to the network via USSD, and the network replies with an acknowledgement of receipt, such as; "Thank you, your message is being processed. A message will be sent to your phone."