Subject Heading Searching:
When using keywords, terms are being searched in multiple areas, making it more likely that irrelevant results will appear. With keywords, you may have to use two or three terms to describe one overall thing.
Subject headings, on the other hand, are searched within only one area in a result record which describes the actual content of the resource. They use one term to encompass all synonymous or analogous terms, reducing the guess work on which term you should be using. Subject headings are also known as a controlled vocabulary. This means that you choose them from a predetermined list of terms.