Definition

MPEG is the famous four-letter word which stands for the "Moving Pictures Experts Groups.
To the real word, MPEG is a generic means of compactly representing digital video and audio signals for consumer distributionThe essence of MPEG is its syntax: the little tokens that make up the bitstream. MPEG's semantics then tell you (if you happen to be a decoder, that is) how to inverse represent the compact tokens back into something resembling the original stream of samples.

These semantics are merely a collection of rules (which people like to called algorithms, but that would imply there is a mathematical coherency to a scheme cooked up by trial and error?.). These rules are highly reactive to combinations of bitstream elements set in headers and so forth.

MPEG is an institution unto itself as seen from within its own universe. When (unadvisedly) placed in the same room, its inhabitants a blood-letting debate can spontaneously erupt among, triggered by mere anxiety over the most subtle juxtaposition of words buried in the most obscure documents. Such stimulus comes readily from transparencies flashed on an overhead projector. Yet at the same time, this gestalt will appear to remain totally indifferent to critical issues set before them for many months.

It should therefore be no surprise that MPEG's dualistic chemistry reflects the extreme contrasts of its two founding fathers: the fiery Leonardo Chairiglione (CSELT, Italy) and the peaceful Hiroshi Yasuda (JVC, Japan). The excellent byproduct of the successful MPEG Processes became an International Standards document safely administered to the public in three parts: Systems (Part), Video (Part 2), and Audio (Part 3).

Pre MPEG
Before providence gave us MPEG, there was the looming threat of world domination by proprietary standards cloaked in syntactic mystery. With lossy compression being such an inexact science (which always boils down to visual tweaking and implementation tradeoffs), you never know what's really behind any such scheme (other than a lot of the marketing hype).
Seeing this threat? that is, need for world interoperability, the Fathers of MPEG sought help of their colleagues to form a committee to standardize a common means of representing video and audio (a la DVI) onto compact discs?. and maybe it would be useful for other things too.

MPEG borrowed a significantly from JPEG and, more directly, H.261. By the end of the third year (1990), a syntax emerged, which when applied to represent SIF-rate video and compact disc-rate audio at a combined bitrate of 1.5 Mbit/sec, approximated the pleasure-filled viewing experience offered by the standard VHS format.

After demonstrations proved that the syntax was generic enough to be applied to bit rates and sample rates far higher than the original primary target application ("Hey, it actually works!"), a second phase (MPEG-2) was initiated within the committee to define a syntax for efficient representation of broadcast video, or SDTV as it is now known (Standard Definition Television), not to mention the side benefits: frequent flier miles