The Stanford University Libraries--like most libraries in the U.S. and even globally--use Library of Congress Subject Headings as their means of categorizing books according to subject in a standardized way (referred to as "controlled vocabulary"). Catalogers assign these headings using prescribed rules for format, but their actual choices of headings are based on their understanding of a book's subject. What this means is that searching on a single subject heading will almost never retrieve all of the books a library owns on that subject. Therefore it's often good to experiment and see what other related headings exist and to look at the headings that have been assigned to a book you want to find more like. The Library of Congress provides a discussion of subject headings HERE.
Tips for searching SearchWorks using subject headings:
If you are reviewing a record in SearchWorks and find one of its subject headings useful, simply use it as a hyperlink. But keep in mind: catalogers often augment subject headings by adding subheadings that make them more specific--e.g., a geographical term, a time period, a format. Where you click on a subject heading's link will determine how much of the compound term is included in your new search.
If you don't know the specific subject you're looking for but have a general idea of what it might be, start by restricting your search to the Subject field. Then pick some terms that you think might appear in the subject heading of an item you'd be interested in.
Try it: If you're looking for books about the depiction of women in Medieval art, simply try typing the keywords "women," "Medieval," and "art" into the Subject search box. Here is the result. Open a few records and look at the subject headings. Notice that often one keyword belongs to one heading, while a second keyword belongs to another. Subject headings tend to work well in combination, since books' topics can be quite complex.