Boolean Operators are words that connect keywords (search terms) together to broaden or narrow the results retrieved. In library research they are often used with the library's research databases or the library catalog.
The three Boolean operators are AND, OR, and NOT.
One way to visualize Boolean Operators is to use a Venn diagram. See below or watch the video to the right.
Take a look at the following sandwich related searches in Academic Search Complete to see how this affects the number of search results generated.