Boolean searching is the use of Boolean logic and set theory. Boolean operators, such as AND, OR and NOT, are used to combine searches in a variety of ways. When you do an advanced search using more than one line, you are actually taking advantage of Boolean searching.
AND --Use the AND operator between two or more words or phrases to find articles containing all of the words. For example, "George W. Bush" and "Laden" will limit results to those items containing both topics. Any article containing just one of the topics will be eliminated.
OR -- Use the OR operator between any two words or phrases to find documents containing either of the two words. For example, "George W. Bush" or "Laden" will bring up every article that contains the topic George W. Bush along with every article that contains the word Laden. The articles on bin Laden do not have to mention George W. Bush and vice versa.
NOT -- Use the NOT operator between any two words or phrases to find documents that do not include the word or phrase immediately following the NOT. For example, "George W. Bush" not "Laden" will only retrieve articles that contain George W. Bush but don't mention the word Laden. Any article that talks about both Bush and bin Laden will be eliminated.
Some databases will have an advanced search that will let you put the Boolean terms on different lines. Others will allow you to search using one line, but putting the operator between the words in ALL CAPITALS. Example: George W. Bush AND Laden.