Library of Congress classification system is what almost all college libraries use, as do we. It is a letter and number system, in which the letter and number combinations denote different subjects. For example, medical books are in the "Rs," and nursing books are in the "RTs." You will see a link to their site in the book section of many of our library research guides, to aid in browsing the shelves.
Pictured here is the man who led the creation of the system, Herbert Putnam, Librarian of Congress from 1899-1939.