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AND tells the computer to return only results with both of the terms entered.
Cats AND Dogs
This would only show you articles or books that have both words. It would not show you results that have only one of the terms.
AND makes your results set smaller, but more specific.
OR tells the search engine that you want results with either of a pair of terms. It does not require that both terms are present. Try to use OR with synonyms or words with similar enough meanings that they are interchangeable.
gasoline OR petrol
cost OR price
OR makes your results set larger.
NOT allows you to exclude a term that is irrelevant to your search.
Saturn NOT Planet (want the automobile, not astronomy)
NOT can be tricky, but it makes your results list smaller and more specific by eliminating what you don't want. If you repeatedly retrieve unrelated articles, you might look for a term that you can use NOT with to exclude them from later searches.