AND - looks for ALL the terms in your search, in any order. It is the default for most databases. If you don't type AND in between your terms, the database will automatically do it.
OR - looks for ANY of the terms in your search. It is used to group synonyms together. Normally when you have a single search box, you put the synonyms within parentheses, e.g. (adolescents OR teenagers OR youth). If you have multiple search lines, you can put the synonymous terms altogether on one line.
NOT - will exclude certain terms, and is normally used at the end of a search statement. For example, pharmaceuticals NOT illegal. Use this operator with care, because it may exclude some relevant sources.
" "- quotes are used to search an exact phrase using two or more keywords.
* - truncate to find any ending of a root word. (teen*, teens, teenager)