Archives are managed by a variety of types of institutions and the materials they collect differ. Familiarity with different kinds of archives is helpful for finding resources that are most appropriate for your research.
Museum archives: maintain diverse collections that typically consist of artwork or artifacts relevant to the type of museum. Items are often displayed. Here's the archives of the Metropolitan Museum of Art
Religious archives: collect and preserve materials related to a faith, denomination, or individual place of worship. Here's the archives of the Archdiocese of Seattle
Special collections: a collection of items, usually in a library, that are either irreplaceable or rare. These items are kept separate from regular library collections. Check out Special Collections at the University of Washington Libraries.