This is a "tree of all knowledge" category, a top-level place to start when browsing Wikipedia categories for articles. This is the top level in terms of encyclopedia article function and content. It is intended to contain all and only the few most fundamental ontological categories which can reasonably be expected to contain every possible Wikipedia article under their category trees. These categories are: physical entities; biological entities; social entities; and intellectual entities.
An alternative root category, based on a somewhat more detailed initial classification, is Category:Main topic classifications.
This is a list of Wikipedia's major topic classifications. These are used throughout Wikipedia to organize the presentation of links to articles on its various reference systems, including Wikipedia's lists, portals, and categories.
Categories are pages that are used to group other pages on similar subjects together. This is done to help users find the pages they are looking for, even if they do not know whether it exists or what it is called.
Every page should belong to at least one category. A page may often be in several categories. However, putting a page in too many categories may not be useful.