The terms "archive" and "special collection" are often used interchangeably. However, in a university setting, they are different things.A university archive is a collection of material related to the history of the university. For an example see this description of the Rutgers University Archives:
The University Archives (9,000 cubic feet) serves as the final repository for the historical records of Rutgers University. Its primary purpose is to document the history of the University and to provide source material for administrative use and for researchers who seek to evaluate the University's impact on the history of American social, cultural, and intellectual development. Included are records of administrative and academic units, student organizations, theses and dissertations, photographs, memorabilia, and personal papers of faculty, students, and alumni.
A special collection collects, preserves, and makes available primary sources of a rare, unique, or specialized nature to support advanced study and research about a particular topic or in particular subject area. For an example see this description of the Rutgers University Manuscript Collection, part of the Special Collections and University Archives:
Containing original letters, diaries, sermons, literary manuscripts, organizational records, financial and legal records, and other documents, the Manuscript Collection . . . is especially strong and widely known for its documentation of all aspects of New Jersey history and society and New Jerseyans' response to national and international issues, 18th century to the present. Other strengths include 19th century Westerners in Japan, 19th and early 20th century British writers, the consumer movement in the United States, 20th century Latin American politics, society and Inter-American cooperation, 20th century labor unions in the United States, American business and technology, American social welfare and social policy, and women's history. (Both quotations from the Rutgers University Special Collections and University Archives Description of Collection.)
Archives exist outside of universities as well. There are state archives, city archives, museum archives, church archives, community archives, and archives devoted to a specific person, movement, or event. These archives contain rare or unpublished material - the "letters, diaries, sermons, literary manuscripts, organizational records, financial and legal records" listed above - but may also collect ephemera, physical objects, or audiovisual recordings.
Some documents in a archive or a special collection may be locatable by online catalog - much like any book in the library system. However, most are not. Often only the most basic information about a collection is available onine. The best way to find out specific information about a special collection or archive is to (a) consult a finding aid, if one exists and (b) to ask the librarian or archivist for assistance.
What is a finding aid? According to the Society of American Archivisits' Glossary of Archival and Records Terminology, a finding aid is:
From the Oregon State University Libraries:
"Finding aids, or collection guides, are descriptive inventories, indexes, or guides that archives create to describe and provide access to the contents of the collections that they hold. These guides are tools that researchers can use to gain access to archival materials. A finding aid functions much like the preface and table of contents of a book, in that it reveals the origin, background or context, contents, and arrangement of a collection. They are often used to determine the relevance of a collection to a specific topic and the parts of that collection that are most relevant to the topic.
A finding aid will usually include general information like the name of the creator of the material, the date span of the material, and the quantity; although you’ll likely find biographical or historical information about the creator and a narrative description or summary of the collection. Generally, you’ll also find information on how the material is organized (it may be broken down into a number of "series") and for many collections the finding aid will include a listing of all the boxes and the folders contained in them (sometimes with a great deal of detailed information on their contents and sometimes just a basic listing)."
San Diego State University has a helpful series of videos on the different parts of a finding aid.
The Society of American Archivists offers a sample finding aid describing an archival collection. This finding aid is annotated in order to define and describe each separate element.
Adapted from the Society of American Archivists and the blog of the American Historical Association.