Connectors (Boolean operators) are used in keyword searches to link together two or more terms. The most commonly-used connectors are: AND and OR.
AND narrows a search; you get fewer records, because both concepts must be present in the records found--here the area in color. Use AND to combine different concepts in one search.
OR broadens a search; you get more records because OR includes each of the concepts separately, as well as both concepts when found together. OR is often used to link together related concepts.
NOT narrows a search; you get fewer records because the second concept must not be part of the records found. Use NOT to limit your results. Be cautious when using NOT because you may inadvertently exclude relevant records from your search.
(Source: Our many thanks to the Reference Librarians at Union Institute & University Library for the use of this portion of their Library Research Guide.)