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.
Categories are normally found at the bottom of an article page. Clicking a category name brings up a category page listing the articles (or other pages) that have been added to that particular category. There may also be a section listing the subcategories of that category. The subcategorization feature makes it possible to organize categories into tree-like structures to aid navigation.
The term category does refer to both the title of a category page—the category pagename—and the category itself. Keeping this in mind while reading about categorization, plus learning a category page layout is a worthwhile investment in research techniques.