Session Description Protocol or SDP describes multimedia communication sessions for the purposes of session announcement, session invitation, and parameter negotiation. It is purely a format for session description -- it does not incorporate a transport protocol, and it is intended to use different transport protocols such as SAP, SIP, RTSP, electronic mail using the MIME extensions, and the Hypertext Transport Protocol. However, it is not intended to support negotiation of session content or media encodings, which is viewed as outside the scope of session description.

Examples of SDP Usage:

• Media Streaming - used in RSTP client and server negotiation of parameters in media delivery.
• Session Initiation - SIP messages carry SDP to allow participants to agree on a set of compatible media types.
• Email and World Wide Web - SDP enables the automatic launching of applicationsfor the www client or mail reader in standard format

Below are useful links that explain this protocol more fully:

RFC4566 - SDP: Session Description Protocol
Wikipedia article on Session Description Protocol