Librarians love to organize information. One way we do this, is by using subject headings. Since some subjects can be described using various terms (like cats or felines), librarians have come up with a standardized list, or controlled list, of subjects that can be used when describing resources. This list (known as the Library of Congress Subject Headings) provides the controlled vocabulary that every librarian uses when they describe the subject of a book, DVD, or any other object in the library's catalog.
The easiest way to understand subject headings is to see them in action. The most convenient example is the library's catalog. When you pull up the full record of a title, it will give you the subject heading for that item hyperlinked, so you can search for other titles with that descriptor.
You also will find subject headings in many of our online databases (like Academic Search Complete), where they help organize thousands of articles. They may not utilize the same list of subjects as the Library of Congress, you can still approach searching them the same way as in the library's catalog.