Boolean operators are used to connect search terms in a way that either deliberately narrows or broadens the search.
The default search for the four major search engines is the Boolean AND; however, it is not necessary to use it between search terms: a search for chronic child malnutrition, is a search for chronic AND child AND malnutrition, and will retrieve a list of web pages which contain all three terms, anywhere on the page.
Adding more terms to a search will retrieve fewer results. According to OneStat.com, most searchers use only two or three words on average in a search. If a fourth term, and a fifth term, and a sixth term, were added to the search, the number of results would be fewer with each additional term—and more importantly, the results should be more focused.
Two related ideas:
When choosing search terms, it is a good idea to favor obscurity. If you have precise search terms and phrases to describe what you hope to find, use those terms. For example, rather than a search for global warming (global AND warming)—how about searching for "climate change" AND “greenhouse gases” AND “Kyoto Protocol”?
As many words in English have more than one meaning, it is a good idea to add one or more terms to the search that clarifies the meaning of the term with multiple meanings. For example, when looking for information about modeling, add clay or fashion to indicate the type of modeling you are searching for.