Boolean Operators link concepts and are used to broaden or narrow your search. Briefly, here's how they work:
AND - finds results with your all search terms. AND narrows your search.
OR - finds results with any of your search terms. OR broadens your search.
NOT - finds results with only one of your search terms. NOT narrows your search.
Take a look at the Boolean Operators guide for more detailed information about how to use this valuable search technique.