The IPTC and Dublin Core are the granddaddy of standards. I believe it was Photoshop 4 that first began to embed IPTC metadata in images as IIM records. The IPTC standard grew out of the old school process of send images “across the wire” between news agencies.
For Photoshop CS2, Adobe, IDEAlliance and IPTC worked together to update the standard to IPTC Core which is based on XMP. XMP is very extensible – meaning it can store many different types of metadata easily, and it is unicode, which means it can easily work with multiple character sets such as Chinese, Japanese, Arabic – very important as the next Olympic Games will be in China!
The IPTC is organizing it’s first Photo Metadata Conference on 7th of June, in Florence Italy in conjunction with the CEPIC conference. Tough gig going to Italy, but I will be there with Peter Krogh and David Riecks to talk about XMP, photographer’s workflows and controlled vocabularies respectively.