It's a system of drawers filled with index cards that list books and other materials held by a library. Each card has information like:
* Author: The name of the author
* Title: The title of the book
* Subject: Keywords describing the book's topic
* Call Number: A unique code that helps locate the book on the shelves
The term "card catalog" might have been used in the past to refer to different kinds of card-based systems, but those would be more accurately described as "catalogs" or "indexes."
For example:
* Subject Catalog: A card catalog organized by subject, similar to a traditional card catalog but only for specific topics.
* Author Catalog: A card catalog specifically organized by the author's name.
* Title Catalog: A card catalog organized by the title of the book.
These days, libraries have largely moved away from physical card catalogs to online systems. These systems are more efficient, easier to search, and offer more features than traditional card catalogs.