What is it
JPEG (Joint Photographic Experts Group) is a lossy compression method standardised by ISO.
JPEG JFIF, which is what people generally mean when they refer to "JPEG", is a file format created by the Independent JPEG Group (IJG) for the transport of single JPEG-compressed images.
Status
The JPEG compression format was standardised by ISO in August 1990 and commercial applications using it began to show up in 1991. The widely used IJG implementation was first publicly released in October 1991 and has been considerably developed since that time. JPEG JFIF images are widely used on the Web. The amount of compression can be adjusted to achieve the desired trade-off between file size and visual quality.
Progressive JPEG is a means of reordering the information so that, after only a small part has been downloaded, a hazy view of the entire image is presented rather than a crisp view of just a small part. It is part of the original JPEG specification, but was not implemented in Web browsers until rather later on, around 1996. It is now fairly widely supported.
Although the "baseline" variety of JPEG is believed patent-free, there are many patents associated with some optional features of JPEG, namely arithmetic coding and hierarchical storage. For this reason, these optional features are never used on the Web.
MIME type
The registered MIME type for JPEG JFIF images is
image/jpeg
More information
The JPEG compression specification was also published by the International Telecommunication Union (ITU) as Recommendation T.81 and is available in PDF. The specification for the JFIF file format is also available, in PDF
There is an informative JPEG FAQ. There is ongoing work in ISO to develop new versions of JPEG which give less visible artefacts when the image is compressed.
There is more information about the "lossless and near lossless" JPEG-LS format (and the closely related LOCO-I format) from HP. No MIME types have been allocated for this format, and it is not clear whether the lossless and the near-lossless variants will have different MIME types.
