Acronym for Motion Picture Experts Group
Name for a number of different multimedia container formats and associated audio/visual CoDecs.
The spec allowed for three different ways of encoding audio, called layers 1, 2 and 3. MPEG-1 audio layer 3 became very popular in the late 1990s as a way of distributing music on the Internet, because its compression allowed a music piece of several minutes' duration to occupy just a few megabytes, which was tolerable to distribute at dial-up speeds while still offering decent audio quality. These files, commonly known as "MP3" files, are essentially MPEG-1 files containing audio but no video.
MPEG-2 extended the MPEG concept to handle a greater variety of resolutions (both low and high) and a newer video codec. MPEG-2 files are commonly found on DVDVideo discs.
MPEG-3 was supposed to be a standard for use in HDTV. It was abandoned when it was realized that these needs could be easily met by either MPEG-2 or MPEG-4. Hence there is no MPEG-3.
An MPEG file contains one or more "streams". Thus, video is one stream, and audio is another stream; even if the audio is stereo with two or more channels, that is still one stream. MPEG files on DVD-video discs can contain multiple audio streams for soundtracks in different languages, as well as "private" streams (in formats not defined by the original MPEG specifications, but by the DVD-video specification) for holding subtitles and trick-play data. Streams are multiplexed, which means that, as the file is read sequentially, you encounter blocks of data belonging to each stream in turn, which are meant to be played at the same time. This allows a player to process all the streams concurrently, without having to continually jump around the file.
More esoteric details can be found in MPEGTerminology.
MPEG-1 doesn't seem to be subject to any licensing requirements. MPEG-2 and MPEG-4 are licensed by MPEG-LA. It looks like licensing for MPEG-4 is less onerous than for MPEG-2, if only because Microsoft's Windows Media Player doesn't include support for MPEG-2 or DVD-Video playback, unless you pay for a third-party codec or upgrade to Vista Home Premium or Ultimate.