Well, OK, we're not sure that the "LC" subject headings and classification system are really all that much fun, although don't tell our cataloger that ;-) But they are rather handy to know about, if you're someone who likes to dig deeply into the catalog, or browse extensively in the stacks. (Our book shelves, or "stacks," hold the biggest collection of books among the four year SUNY colleges by the way...)
Basically the Library of Congress scheme is a framework with which to organize books; the Dewey Decimal system used in public and school libraries has a similar goal.
Library of Congress classification scheme here. Shown here is Herbert Putnam, creator of this classification system, which has been used for well over 100 years.