Based on George Boule's algebraic system of logic, Boolean operators allow the combination of words to refine a search. Boolean operators are and, or, and not.
Using and between words requires that both words must be present somewhere in the same document or result. The words could be right next to each other in the text, or separated by pages. They just both have to be present in the same document. Sometimes and is the default operator, meaning it is implied when a space is between words. Google's default operator is AND , so when you put a space between words, both words need to be somewhere in the document (and yes, in Google, operators need to be typed in caps).
Or is a good operator for synonyms, for it requires either word to be present in a result. Google uses OR, too. For it to be recognized as an operator on Google, it needs to be capitalized - OR.
Requires that a word not be present. Some databases use and not to express this concept. Use carefully, since it could easily exclude meaningful and relevant material. In Google, to exclude a word, type -word. Example: dogs -cats
Boolean operators can be combined using nesting-grouping words that share an operator to make sure that the operators work they way you want. If you search cats or kittens and dogs or puppies, it may be unclear which terms share which operators. You can control this by using parenthesis (easier than remembering which operators take precedence in databases):
(cats or kittens) and (dogs or puppies)
That is a much more meaningful search than what might happen if and was the operator that a database acted on first, followed by the or operator:
cats or (kittens and dogs) or puppies
Many databases will provide opportunities for nesting in their advanced search options, but without you needing to use the parenthesis. For example this search in Proquest:
is actually processed as: