UDP
Usergram Data Protocol, an unreliable and unstreamed transmission packet format over TCP/IP networks. To describe what UDP is, it's easier to explain in terms of what it isn't -- and that's best explained by describing the guarantee that its counterpart, Transmission Control Protocol, provides. In TCP, each packet of data is given the following guarantee: if it arrives, it will reliably arrive in order (no data will be lost from the stream before it, and no data from the stream which was sent after it will be delivered to the application before it).
In UDP, no guarantee that it will be delivered at all, in any given order, or in any given timeframe is provided by the network layer. This may sound like a negative, and for many purposes it is -- but it also uses a little less than one half of the packets used by TCP. This lower number of packets makes it the only useful choice in situations which are time-sensitive, including streaming audio and video. This lower number of packets also makes it suitable for situations where not every packet must arrive perfectly intact in order, including DNS (name to address resolution on the Internet) and Second Life.
For more information on TCP and UDP, please see their respective definition documents. (Note that these protocols use the services provided by the IP layer underneath them, be it IPv4 or IPv6.)