Most libraries use some sort of call number scheme to organize their books and make it easier for patrons to find materials.
The two most common call number schemes are the Dewey Decimal Classification scheme, which is used by a majority of public and K-12 school libraries, and the Library of Congress Classification scheme, which is used primarily by local government and academic libraries. There are other schemes that are used by special libraries, like medical and federal government libraries.
Some libraries are even moving towards a bookstore way of organizing the books on the shelves and may be getting rid of their call numbers altogether.
Many of these resources and programs are funded under the provisions of the Library Services and Technology Act from the Institute of Museum and Library Services.