Standards: XMP
The Extensible Metadata Platform or XMP is a specific type of extensible markup language used to store metadata in digital photos. Adobe introduced the format in 2001 when it released Photoshop 7.
Adobe, IPTC and IDEAlliance then collaborated in 2004 to introduce the IPTC Core Schema for XMP. It transfers metadata values from IPTC headers to the more modern and flexible XMP format.
A unique advantage of XMP is that it allows creation of custom metadata panels. These not only store additional forms of data, but also organize it differently from Photoshop’s "File Info" defaults. These XMP-based panels can be installed in Photoshop (see http://www.adobe.com/products/xmp/custompanel.html for details), and allow anyone to insert custom metadata in image files.
Understand, however, that while you can add custom information this way, only Adobe Photoshop and Bridge, along with a few other image databases can import or see these metadata. And custom panels require additional set up work before others can use them.
Deeper Reading:
XMP is a combination of XML and RDF. XML is like HTML. But while HTML focuses on the presentation of data, XML is concerned with “representation.” Additionally, XML is non-proprietary, operating system independent, fairly simple to interpret, text-based and cheap. RDF is the WC3's solution to integrate a variety of different applications using XML as an interchange syntax. Some uses include library catalogs, worldwide directories, news feeds and software, as well as as collections of music, images and events.
Together, the specifications provide a method that uses a lightweight ontology based on the Dublin Core. It also supports the “Semantic Web” (allowing for easy exchange of knowledge on the Web).
A good comparison for how the Dublin Core metadata fields compare to the IPTC fields was on a Dartmouth University site: http://www.dartmouth.edu/comp/about/projects/digitalmedia/images/resources/metadata-schemas.html (now linking to the page on the Internet Archive Wayback Machine).