A quick overview of Boolean search terms and their effects: 

Terms	Effect	Example
AND (+)	Searches for documents containing both words.	Searching for dog AND bone finds pages where both dog and bone occur somewhere in the page or use +dog +bone
OR	Searches for documents containing either word.	Searching for dog OR bone finds pages where either dog or bone occurs somewhere in the page.
NOT (-)	Eliminates documents containing the word after "not" in the search phrase.	Searching for dog NOT bone finds pages where the word dog (only) appears but eliminates pages with the word bone.  Note that even if dog is on the same page as bone the page will be rejected, so some instances of the desired word could be missed.
COMBINED	Performs a combination(s) of the above effects.	For example, searching for dog AND bone NOT chicken finds pages where both dog and bone occur somewhere in the page, but eliminates any pages with the word chicken. (Useful if you don't want to consider chicken bones, which are bad for dogs, for example). Other combinations and associated results are possible.