An English mathematician by the name of George Boole developed a type of algebraic logic that is the basis of most computerized searching that we use today. It is used to search in databases, and it is also widely used by Internet search engines. A basic understanding of Boolean logic will help you to understand what is going on no matter where you seek electronic information.
Boolean logic involves understanding how sets of things overlap.
If you type the search statement CATS and DOGS you will retrieve all items containing both words, but no items containing just one of them. This is illustrated by the shaded portion of the diagram below. Cats Dogs You can see how this narrows down the number of items retrieved. If you type the search statement CATS or DOGS you will retrieve any item that contains either word, even if it does not contain both. The shaded portion of the following diagram illustrates this relationship.