Boolean logic symbolically represents relationships between entities. There are three Boolean operators:
AND Use the AND operator to retrieve a set in which each citation contains all the search terms. This operator places no condition on where the terms are found in relation to one another; the terms simply have to appear somewhere in the same citation.
OR Use the OR operator to retrieve documents that contain at least one of the specified search terms.
Use OR when you want to pull together articles on similar subjects.
NOT Use the NOT operator to exclude the retrieval of terms from your search.
Originally copied from PubMed Tutorial.