A call number tells you where to find a book in the library. Basically, it is the book's "address". Call numbers ensure that books on the same subject are shelved near each other.
Each call number begins with one or two letters representing a broad class of knowledge: for example, B = Philosophy, BF = Psychology, HM = Sociology, PR = Literature, and so on. These classes are further subdivided by numbers representing more specific topics. Thus, a book about English romantic poetry may have the call number: PR 571 F79 2011.
The call number for a book found in the library catalogue is matched to a label on the book's spine (or, for thinner books, the front cover):
For more on call numbers, look here.