RTP, or the Real-Time Protocol, is a standard protocol for transportation of data in real time. It's most commonly used for audio and video, but it can be, and has been, applied to other things, such as interactive games, media-on-demand, IP telephony, real-time remote control applications, distributed simulations, etc.
RTP consists of two parts, a data portion and a control portion, specified separately as RTCP; the Real-Time Control Protocol.
The protocol provides support for applications with real-time requirements, for example continuous media (audio and video), and various related services, such as content identification, loss and error detection, timing reconstruction and security options. The current spec does not address issues such as resource reservation or quality of service, though. For those services, RSVP can be deployed.
RTP is most commonly implemented over UDP, but efforts have been made to make it transport-independant, so that any protocol could be used. It has successfully been used over IPX, CLNP and ATM networks.
RFC 1889: RTP: A Transport Protocol for Real-Time Applications
RFC 1890: RTP Profile for Audio and Video Conferences with Minimal Control