The principle of Boolean logic lets you organize concepts together in sets. When searching computer databases, including keyword searching of the online catalog, these sets are controlled by use of Boolean operators OR, AND, and NOT.
But forget about libraries and computers for a moment and think about ice cream. Imagine all the possibilities a soft ice-cream machine could make if it offered chocolate, strawberry, and vanilla, and could mix together any and all combinations of those flavors. There are seven possible combinations of ice cream flavors available: each flavor by itself, three combinations of two flavors in a swirl, plus all three flavors mixed together.