From the David L. Rice Library, University of Southern Indiana.
Controlled vocabulary--also known as subject headings, subject terms, thesaurus terms, or descriptors--are the official indexing terms used by the organizers of a database to describe each concept so that all items on the same topic have the same descriptor or subject heading.
In contrast to “free Web” searching, controlled vocabularies organize the information in a database. You can click on a heading to see all the other items that have the same heading. You can also combine the terms in a new search to find items on that topic.
Your search results will be more focused and more relevant, since you will be searching directly in the subject or descriptor field.