Boolean logic is a system of showing relationships between sets by using the words "AND," "OR," and "NOT." Boolean logic is recognized by many electronic searching tools as a way of defining a search string. So, Boolean operators AND, OR and NOT can refine your searches on Google (or any other search engine) by combining or limiting terms.
Here are some examples of boolean search strings:
mushing AND racing
caribou OR reindeer
fisheries NOT Alaska
Sets get smaller the more " AND" is used, and larger the more "OR" is used. Although you might think "AND" would add more hits to a set, what it has actually added is limitations. The term "OR" adds possible options.