http://www.adobe.com/products/xmp/main.html Adobe XMP: Adding intelligence to media Adobe’s Extensible Metadata Platform (XMP) is a labeling technology that allows you to … With XMP, Adobe has taken the “heavy lifting” out of metadata …

http://www.adobe.com/products/xmp/overview.html Adobe XMP Overview
Extensible Metadata Platform (XMP). Overview: Metadata and XMP. Metadata is any
data that helps to describe the content or characteristics of a file. …

http://graphicssoft.about.com/od/glossary/f/metadata.htm What is Metadata? About XMP Metadata used in Graphics Software 

http://dl.extensis.com/downloads/PF/EN/P/Portfolio_XMP_Workflow.pdf Using XMP to Drive Your Workflow About XMP Support in Extensis Portfolio

http://www.xml.com/pub/a/2004/09/22/xmp.html (XMP Lowdown by Bob Ducharme)
The Extensible Metadata Platform is a specification describing RDF-based data and storage models for metadata about documents in any format. The specification includes information about embedding XMP in text files such as HTML and SVG/XML; image formats such as JPEG, TIFF, and GIF; and Adobe formats such as Illustrator, Photoshop, and Acrobat files. While its use of the RDF metadata format comes with some caveats, RDF enthusiasts should be pleased to see that the company making the most popular image manipulation tools has made those tools capable of reading and writing RDF metadata about these images.

http://xml.coverpages.org/xmp.html (Extensible Metadata Platform, XMP, Cover Pages)
The Cover Pages document the progress of XMP as “news,” and provide a dedicated topic document on XMP.

http://www.thirdlight.com/appliance/faq/keywording (Keywording Images, Thirdlight)
Metadata helps to describe the contents of a file. Basic metadata is often seen in the File Info or Document Properties box found in many software applications, eg. PhotoShop. Many asset management systems captures some metadata and display this to you. In modern operating systems, meta data is used to allow desktop searches.

http://www.reallysi.com/newsletter18_2.htm Adobe’s New World Order: Metadata-Driven Workflows with XMP, Lisa Bos
In 2001, Adobe published the specification for XMP, the eXtensible Metadata Platform. . .  At first glance, this platform might not seem revolutionary. But, in fact, Adobe is attempting to fundamentally change the way we think about asset management processes in a creative environment.

http://www.ideum.com/xmp.html 7 Steps to Understanding XMP (Flash Tutorial)

http://www.poundhill.com/resources.html  XMP Information, Resources and Links (Poundhill whitepaper)
What is metadata? Put simply, it’s any data that describes the content or characteristics of a file. You are probably already accustomed to viewing basic metadata through the File Info or Document Properties box found in many applications. You may also use an asset- or content-management system that captures some file properties and displays them for you.