From Forum Nokia Wiki

USSD stands for Unstrcutured Supplementary Services Data.

• It is a way of sending short commands from the mobile phone to the GSM network.

• It uses, like SMS, the signalling channel of the GSM connection. Unlike SMS, it does not use a store and forward architecture, but a session oriented connection.

• USSD text messages can be up to 182 bytes in length. Messages received on the mobile phone are not stored.

Links

• 3GPP TS 03.90 Unstructured Supplementary Service Data (USSD) Stage 2
• 3GPP TS 22.090 Unstructured Supplementary Service Data (USSD) Stage 1