Boolean operators are based on algebraic system of logic formulated by George Boole (1815-1864), the British mathematician (if interested check out his bio here ). You use the operator (aka conjunctions) AND, OR , NOT to narrow or broaden your search. You don't need to capitalize the operators; it's just a librarian thing to do! ;-)
AND -- is the intersection of terms; AND narrows your search. It tells the database that ALL words must be present in the results
OR -- is the union of terms; OR broadens your search. It tells the database that any of the terms can be present in the results. Think of synonyms or similar terms when you are OR - ing your terms.
NOT -- is the elimination of terms. NOT narrows your search but it ignores certain terms. It tells the database to ignore specific term or concept in the results.