Keyword searching looks for the exact form of the keyword somewhere in the record. Subject searching also uses keywords, but the subject keywords assigned to a source depend upon a classification of the source's content that has been determined by the database and that employs a limited vocabulary of possible subject terms. Since this vocabulary varies from database to database, you will need to identify the relevant subject headings for each database you search in so that you can maximize your chances of finding sources relevant to your research.
Click below for some tips for identifying potentially useful subject headings:
Finding Subjects in the Fenwick Library online catalog
Finding Subjects in the Expanded Academic ASAP Database
Finding Subjects in the Modern Language Association Bibliography