Are words and phrases which constitute a "controlled vocabulary" to categorize books by subject field. Subject headings often indicate the contents of books in terms that their titles do not use, which often may be very general.
Subject headings in online databases are often referred to as descriptors, but they serve the same purpose in locating valuable resources. Using a "keyword" search will identify catalog entries that contain those specific terms which can add value to a search; however, the terms can be found anywhere - author, notes, publisher, etc. - therefore there is no indication of the value of the term(s) as content matter of the items in a results list.
Critical works are classified and sorted by author's name, dates of birth and death, and the subheading, Criticism and Interpretation.
This usage works on most databases, but check 'Help' sections for supported search syntax.
art* (asterisk as truncation character) searches for art, arts, artistic, artful, etc.
m?n (question mark as wildcard character) searches for man and men.