The first coordinated metadata effort was the Information Interchange Model, developed by IPTC in the early 1990s.
The latest version of the spec I could find is here: http://www.iptc.org/std/IIM/4.1/specification/IIMV4.1.pdf
It had its largest acceptance among photographers, as the standard for embedded metadata in images.
XMP, the Extensible Metadata Platform, is a successor, but its vocabulary is still based on the IPTC schema (because XMP was a collaboration between Adobe and IPTC).
The other main metadata initiative is more encompassing, the Dublin Core Metadata Initiative. The document specifying metadata terms is found here:
Here’s a quicky description of everything: http://www.controlledvocabulary.com/imagedatabases/iptc_naa.html
And an online Exif viewer to boot: http://regex.info/exif.cgi
And an example of its use: http://regex.info/exif.cgi?url=http%3A%2F%2Fregex.info%2Fi%2FJF4_008756.jpg