IPTC
A metadata standard developed for the press, holding descriptive and rights information such as caption, creator, keywords and copyright.
In depth
IPTC is the language picture desks and agencies speak: a correctly filled caption, credit and copyright field travels with the file through every system that respects the standard. For anyone licensing images it is not optional, since an unlabelled file circulating without a creator is effectively orphaned.
Examples in practice
A news agency rejects submissions whose IPTC caption does not name the subject, location and date.