Help:Category
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Categories are intended to group together pages on similar subjects. They are implemented by a MediaWiki feature that adds any page with a text like [[Category:XYZ]] in its wiki markup to the automated listing that is the category with name XYZ. Categories help readers to find, and navigate around, a subject area, to see pages sorted by title, and to thus find article relationships.
See the full documentation on 
Wikipedia's help page on categories.