The Zombie Stack Exchanges That Just Won't Die
Whoever asks this, regardless of context, is or has been a patron of a library that used DDC, so phrase the answer like you're addressing a patron.
(Aigh. I can try? Supposedly I teach this stuff...)
Classification systems like DDC exist to accomplish two major goals:
DDC classifies books by their main subject -- what each book is about. To make it possible to classify any book that might come into the library, a classification system has to encompass as much of the total universe of knowledge as libraries generally hold -- which is a large universe!
DDC was designed by 19th-century American Melville Dewey, and is used to organize the nonfiction shelves in most American public libraries. (You may have noticed that much fiction in American libraries doesn't use DDC; instead, it's often arranged by genre -- romance, mystery, speculative fiction -- and author.) If you think the way Dewey split up the knowledge universe:
is a trifle odd, you're not alone! It's a reflec-century educated-gentleman sensibilities (well, except for "computer science," which is obviously a later addition), and things have changed a good bit since then.
When a new book comes into the library, a librarian whose specialty is cataloging skims the book to determine what it is about, then looks that subject up in a Dewey "schedule" (actually some rather thick books or a complex website) to work out what its Dewey call number should be. Once labeled with that number, the book goes to a shelf near neighbors on the same or a similar subject. Catalogers in different libraries may make different decisions about where to classify a book, depending on their sense of where the book best fits with the local library's collection.
The trademarks and other intellectual property associated with DDC belong to OCLC, Inc., which publishes revised Dewey schedules.
Attempt at a shorter answer:
The Dewey Decimal system is like an address system for books. When you go to visit someone, you look at their town and then address.
Looking at an example of Dewey Decimal 123.45.
When I teach kids to find things in Dewey, I tell them the first part of the number is the street address, and the part after the dot is the apartment number. So we wander over to the shelves together until we find the right street address, then we start looking for the right apartment number, digit by digit. If there are a lot of books with that apartment number, then we have to look at the names on the mailbox, on the line below. Obviously I point out that all the books in the area have a similar subject, and then I tell them that if they go to the same address in the adult section, they will find more books on the same subject! If it is appropriate, I show them.
If I still have their attention, I will ask them if there is another subject they are interested in, and I will take them to that address. In the end, it is less about explaining the system than it is about leading them into discovery of it.
My approach with adults is often similar. I don't try to teach the whole system, but start with the part they are interested in... "Dewey puts the travel books in the 900s, where they are grouped by part of the world - see how Asia is all together, and here is North America..." People are generally able to extrapolate from this that different subjects have their numbers. I find most people don't care to learn the whole range, just their areas of interest.
This was told to me a long time ago, and I'm a little hazy on the details, but a clever mind can modify it and make it their own. Obiously it needs a little tweaking, and I'd love to see how it could be clarified!
"In the beginning, people came into being and that led to the 000s or the Generalities.
Then, people became aware and began to wonder about themselves and others, and that led to the 100s, Philosophy and Psychology.
As next, their thoughts turned to who made them and where did they come from, leading to the 200s, Religion.
As they explored the world and people around them, the 300s, the Social Sciences, became relevant, followed by the 400s, Languages, which allowed them to communicate with others.
An exploration of the natural world around them, the 500s, Pure Sciences, came next, then how people were able to shape the world to their needs, the 600s, Applied Sciences.
As people developed their intelligence and mastered their environment, they had leisure and time for the 700s or Recreation.
People had stories to tell about their lives and the world of their imaginations, which led to the 800s, Literature.
Lastly, people reflected on scope of the world around them and wanted to make a record of their achievements, leading to the 900s, History and Geography."
Sorry for the lack of lyricism and detail but I wanted to roughly outline this flowing narrative. Hope it helps!