Libraries collect and preserve the graphic records of human experience.1 Not every experience is documented, and not every documented experience is collected and preserved. How, then, can you know what part of the historical record remains for you to study? Reference sources are your map to the graphic records of human experience.
Even if every human experience hasn't been documented, and every document hasn't been preserved, there remains a plethora of sources available for the historian to study. Here at the University of Illinois Library, you have access to over 14 million printed books, 9 million microforms, 120,000 periodicals, 148,000 sound recordings, 1 million audiovisual resources, 280,000 e-books, 29,000 cubic feet of archival records, 3 terabytes of electronic records, and 650,000 maps—that's over 24 million potential primary sources for your research. Reference sources are your map to the Library's collections.
Half of one aisle at the University Library's remote storage facility on Oak Street.
Use reference sources for factual information like the date the Defense of Marriage Act was signed into law, or where in the United States Statutes at Large to find it. You can also use them to find more complex information, like marriage law in Islam. They often include bibliographies, making them profitable places to begin a research project.
The purpose of bibliography is to list documents, usually published documents like books and articles. This type of bibliography is more accurately called "enumerative bibliography". An enumerative bibliography will attempt to be as comprehensive as possible, within whatever parameters established by the bibliographer.
Bibliographies will list both secondary and primary sources. They are perhaps most valuable to historians for identifying primary sources. (They are still useful for finding secondary sources, but increasingly historians rely on electronic resources, like article databases, to locate secondary sources.)
Think of a bibliography as a guide to the source base for a specific field of inquiry. A high quality bibliography will help you understand what kinds of sources are available, but also what kinds of sources are not available (either because they were never preserved, or because they were never created in the first place).
For more information on the role of bibliography in historical research, see our guide to Bibliography and Historical Research.
Catalogs are similar to bibliographies (see above), and the two are often grouped together, and can be used in similar ways and for similar purposes (discover of sources). The essential difference between bibliographies and catalogs is that former will list any documents known to have existed, while the latter will be lists of sources that are held by a particular library. Like bibliographies, many catalogs are now available online.
1. Jesse H. Shera, The Foundations of Education for Librarianship (New York: Becker and Hayes, 1972): 193.