AND will allow search terms to be combined so that all results include all search terms in the strategy.
OR allows searching with synonyms or multiple words at one time. All search results will include every word in the search statement.
NOT and AND NOT allows a word or concept to be eliminated from all search results. It must be used carefully to be sure a relevant item is not dropped from the search results.
NOT and AND NOT do the same thing, but some databases require NOT and others require AND NOT. Check the help file in the database if the choice is not obvious.
** The NOT and AND NOT operations should be used with extreme caution. It is very easy to eliminate useful materials by adding a NOT phrase because article frequently address more than one topic. **