A system of classifying books and other library materials developed and maintained over the last 200 years by the Library of Congress in Washington, D.C. In LCC, human knowledge is divided into 20 broad categories indicated by single letters of the roman alphabet, with major subdivisions indicated by a second letter, and narrower subdivisions by decimal numbers and further alphabetic notation.
Example:
LC call number: PE 3727.N4 M34 1994 In the example given above (assigned to the book Juba to Jive: A Dictionary of African-American Slang edited by Clarence Major), P represents the main class "Language and literature," PE the class "English language," 3727 the subclass "English slang," and N4 African Americans as a special group. M34 is the Cutter number for the editor's surname and 1994 is the year of publication. In the United States, most research libraries and academic libraries use LCC, while most school libraries and public libraries use Dewey Decimal Classification (DDC). Click here to see an outline of LC Classification and here for the LC Classification Weekly Lists.

From ODLIS, Online Dictionary for Libary and Information Science by Joan M. Reitz
http://lu.com/odlis/about.cfm

Click here to browse the main classes of the Library of Congress Classification