George Boole, 1815 - 1864
George Boole was an English mathematician who helped establish the modern field of mathematics called symbolic logic. Boolean logic uses words called operators. The three main operators are: AND, OR and NOT.
Computers actually use a type of Boolean logic in their electronic circuits, but where this logic is most useful to your research needs is that databases can use Boolean's logic to locate items from your search.
The blue areas in the following diagrams represent the number of hits you would receive from doing a search using the Boolean operators AND or OR in the same database:
using OR retrieves a large number of items: