Originally designed to allow press photographers to provide information concerning images they sent back to the home office, IPTC (International Press Telecommunications Council) fields can be used to store supplemental information about an image. IPTC information differs from EXIF information in that EXIF information is generated by the digital camera and contains information about various camera settings. IPTC fields can be used to �annotate� images with a great deal of information.
Further info...
http://en.wikipedia.org/wiki/IPTC