A keyword search retrieves any and all occurrences of a given word or combination of words, whether the words appear in the subject headings, title, or descrioption of the article or book.
Advantage: A keyword search expands the search to all occurrences of a given word or combination of words.
Disadvantage: A keyword search may retrieve much more than you want.
A subject search searches the subject headings assigned to articles and books. Subject headings are very specific terms and phrases used by libraries to describe what a book or article is about. In order to have these terms and phrases be consistent, most libraries use subject headings defined by the Library of Congress. A subject search limits your search to an exact word or an exact combination of words assigned to a given article or book as subject headings.
Advantage: A subject search retrieves exactly what you ask for.
Disadvantage: A subject search limits you to only what you ask for.