The Library Catalog provides bibliographic information about the books, periodicals, videos, databases and other collections owned by a library. In the past, these records were kept on 3 X 5 cards in the wooden drawers of a card catalog. Today card catalogs are pretty rare: most libraries have converted to electronic databases searchable via the Internet.
Regardless of the form of access, the function is the same: to describe where the materials contained in a library collection are located and to make that information accessible through either an author, title or subject search.
Each item in a library catalog is identified by a call number that explains its physical location. These numbers are carefully chosen so that books and materials on the same subject will end up next to each other on the shelf. College and university libraries generally use the alpha/numeric Library of Congress classification system while local community libraries generally use the older and more familiar numeric Dewey Decimal system.
Libraries that predate the Library of Congress system will often use both. In any case, devote some research time just to browsing the stacks. You will find many great resources sitting on the shelf right next to the ones you found while searching the catalog. Think of it as an informational treasure hunt: you will be surprised at the resources available that you may otherwise never have discovered.
Certain subjects are not clearly included in either system-some fields, like computer science, mass communications, and environmental studies are newer than the classification systems themselves and so will be found sandwiched into other, related areas. Again, ask your librarian for help when you are unsure how materials are categorized or where to look for them.