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.
Wikipedia is a terrific knowledge resource, and many recent studies in artificial intelligence, information retrieval and related fields used Wikipedia to endow computers with (some) human knowledge. Wikipedia dumps are publicly available in XML format, but they have a few shortcomings. First, they contain a lot of information that is often not used when Wikipedia texts are used as knowledge (e.g., ids of users who changed each article, timestamps of article modifications). On the other hand, the XML dumps do not contain a lot of useful information that could be inferred from the dump, such as link tables, category hierarchy, resolution of redirection links etc.
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.