The difference between keywords and subject headings

A keyword search looks in every part of the record and will retrieve those items that have the words you typed in regardless where they are. In fact, the words don't even have to be next to each other. For instance, if you typed in visual communication, one of the items you would retrieve is this one:

This is a picture dictionary, not a book about visual communication, or using images to teach, explain, impart or inform. Since the catalog looks in the title, author, notes, and subject heading areas, many times, keyword searches will bring up false hits like this one.

How do subject headings work?