Archives are usually unpublished, primary source material that document the activities of an individual or organization. These unique materials are preserved in an archival setting because the information contained therein has enduring value and /or because they provide evidence of the role and activities of the individual or organization that created them. Archival materials that document the activities of an individual are often referred to as manuscripts.
For a more in-depth discussion of the definition of archives and special collections, visit the web site of The Society of American Archivists. There, you will find an an online glossary of archival terms, including a discussion of the definition of archives.