Used with permission. © Helena Söderberg (http://www.flickr.com/photos/craftcreativity/3539093117/)
How do you organize a collection of stuff? In this case, books?
There are many ways you can organize books, by color, by size, or by subject. Public libraries and high-school libraries often organize their books using the Dewey Decimal system.
The Library of Congress system is the standard system for most college and university libraries. Below are a couple links if you want to learn more about this system.
The Courtright Memorial Library uses both the Library of Congress system AND the Dewey decimal system. The library owns a lot of children's books, and children's books are traditionally organized by the Dewey decimal system. The library uses Library of Congress for most other material.
The most important thing to understand about the library's systems is that similar things are shelved together--if you find a great book on the shelf, check the books around it for more about the same topic.
Subject headings are a lot like tagging. However, the headings have been selected and agreed upon. That means, if you find a subject heading that works, this can help open up more options.
Basically, it means that everyone agrees to call something the same thing. So what is this pictured?