Keywords are the words we typically use when we use a search engine like Google. A keyword or keyword phrase are words that describe what we are looking for.
Subjects or subject headings are words that are assigned to each item (such as an article) in a search database. They are similar to tags. The advantage to searching by subject is that the search is much more focused and precise, therefore relevant.
The handout below discusses keywords vs. subject searching in more detail and tells you how to find subject terms.