Business

This will narrow your search to include all terms that are connected by this term. For example, the following requires that both terms be present in the title, abstract, or keywords of a citation.

"student engagement" AND leadership

This broadens your search to include more options based on your search terms. For example, the following insists that either the term college or the term university must be present along with the phrase "student housing" in the title, abstract, or keywords assigned to a citation.

(college OR university) AND "student housing" 

This excludes a term from a search and thus helps you narrow your search results.

You may find that a search on (student AND vulnerability) brings up too many citations related to physical vulnerability. You could add the following to the search in order to exclude articles focusing on that one aspect of the concept of vulnerability.

(student AND vulnerability) NOT physical