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.
Subject Heading Searching
Terms that data uses to describe itself
Terms that describe what the data is actually about
Searches in multiple areas - title, abstract, and full text
Searches only in the subject heading field of a result record
Results lists are broader and can contain irrelevant results
Results lists are usually shorter and more focused
Can include any term in combination
Chosen from a predetermined list and cannot make changes to them
May need to use multiple analogous terms
One subject heading can cover multiple, analogous terms