"... JPEG/Exif is the most common image format used by digital cameras ...;
... JPEG/JFIF, is the most common format for the World Wide Web.
... simply called JPEG "
original Jpeg file format: 'JPEG Interchange Format' (JIF)
"... this "pure" file format is rarely used, primarily because of the difficulty of programming encoders and decoders that fully implement all aspects of the standard and because of certain shortcomings of the standard...
--> JFIF, EXIF
" ... In practice, most JPEG files in Exif format contain a small JFIF header that precedes the Exif header. This allows older readers to correctly handle the older format JFIF header, while newer readers also decode the following Exif header. ... "
"Syntax and structure
A JPEG image consists of a sequence of segments, each beginning with a marker,
each of which begins with a 0xFF byte followed by a byte indicating what kind of marker it is.
Some markers consist of just those two bytes; others are followed by two bytes indicating the length of marker-specific payload data that follows ... "
"... A freely-available tool called packJPG is based on the 2007 paper "Improved Redundancy Reduction for JPEG Files." There are also at least two companies selling proprietary tools with similar capabilities, Infima's JPACK and Smith Micro Software's StuffIt, both of which claim to have pending patents on their respective technologies. ...