Although it was developed to organize materials housed at the Library of Congress, the Library of Congress Classification system (LCC) is now widely used in academic libraries around the world. The LCC system begins with 21 categories, each identified by a letter of the alphabet. Most of the categories are also broken down - to get more specific - and identified by 1 or 2 more letters. For example N contains Fine Arts; ND is Painting, a topic in fine art.
Individual topics are also delineated by place, time period, etc. using numbers (in ND673 you'll find books on painter Peter Paul Rubens who lived 1577-1640). A call number may have up to 4 digits (1-9999) in the main part, and then if necessary, decimal places that may have only numbers or both a letter and numbers. (Library of Congress Classification http://www.loc.gov/catdir/cpso/lcc.html)
Below are the major classes or categories found in LCC: