IPTC in IMATCH
An acronym for [International Press & Telecommunications Council]. IPTC News Codes have their origins prior to the days of digital cameras and date back to the era of wire machines. IPTC (as it is generally referred to) simply describes a set of metadata tags that can be embedded in an image file once it is separated from the camera and is being read by an appropriate application. IPTC tags are used to contain metadata about the subject of the image, the creator of the image, location, copyright, keywords, etc. (just open the IMATCH IPTC editor to see all available fields).
Unlike EXIF data, which is automatically written to the file by the camera when you take a photograph, IPTC is manually added and amended by anyone with access to the image file. Also, because the metadata is embedded directly in the image file so you can never lose it unless you delete, or lose the image itself.
If you intend to market your images then IPTC is currently the protocol of choice as it is as close to an industry standard as there is and will likely be used by publishing houses for some time to come. This will mean using IPTC mostly in the way it is described by the IPTC, but also accepting that publications and image libraries do also use tags in "non-standard" ways and tailoring to meet that need when required. [Example] One major newspaper group won't give a credit if they publish a image unless the photographer's name is in the Byline tag; another national magazine will only credit the image in the magazine if the Credit tag is used.
If you don't intend to market your images for publication, then there are a number of interesting things you can use IPTC tags for instead. However, it is STRONGLY recommended you learn about IPTC and its intended usage first, then you can decide how you would want to use it in other esoteric ways. For example, IPTC can be used to "store" (part of) the IMATCH Categories assigned to an image. This info can be used to recreate the assigned categories in another database, or rebuild the category tree in a new database.