Dewey was born in Adams Center, just southwest of Watertown (where our own Mary Hoffman once lived). He was 26 when he conjured up the Dewey Decimal System. This was a means of dividing all human knowledge into 10 broad categories, which could be divvied up further still depending on the subject (like biography, which begins with "920"). Before then, librarians were expected to arrange books on the shelves by either size or color, and memorize the placement of each one.
While we've never used DDC at this library (shoehorning some of the subjects in our collection into the rigid ten categories results in some astronomically long classification numbers), it is something that's found in most public and school libraries.
You've all heard of it. To this day, I get to hear cracks about whether I arrange my clothes per the Dewey system (no . . . I use the Library of Congress method). That's not Melvil's fault, though. Happy birthday, old man.