Answer:
Short answer: everything
Long answer: everything
Gray area: it depends!
Essentially an archive is established to preserve memory; of an institution, a nation, or even of a person. This is done by collecting, arranging, preserving, cataloguing, and organizing materials relevant to that entity's existence. And my materials, I mean ANYTHING. Fashion designers have archives, the U.S. Government, of course, has one (actually, a few) famous personalities tend to have their papers and artifacts housed, really anyone or anything can have an archive.