Categories are groups of articles on similar topics.[note 1] At the bottom of an article, you will see a box containing the categories to which that article has been assigned. Simply click any of these categories to go to the corresponding category page. From there, you can browse other articles on the same topic, or navigate through the category tree to find other related categories.
How are categories organized?
Most categories have a number of other categories listed as subcategories. These are listed in a section of the category page, which appears above the list of articles belonging to the category. Click on these subcategories to navigate through the category tree to find more specific groups of articles.
You may want to navigate in the other direction, to find more general groups of articles ("parent categories"). These are listed in the box at the bottom of a category page, just as on a Wikipedia article page.
Why is an article not in the categories I would expect?
Articles are not usually placed in every category to which they logically belong. They will not be placed directly into a category if they belong to one of its subcategories. This is because categories would become too large, and the list of categories on articles too long.
To find the articles you are looking for, it may be necessary to dig down. For example, you won't find Oslo listed at the category called Cities, but if you start from there and click "Cities by country", and then "Cities and towns in Norway", you'll arrive at the right place. Conversely, if you are at the Oslo article and you want to find the category of all cities, start by clicking Cities and towns in Norway and navigate up the tree to its parent categories.
What types of categories are there?
The main types of categories used are:
Administration categories or project categories – categories used mainly by Wikipedia's editors for project management purposes, rather than for browsing. Category:Wikipedia backlog links to many categories with articles that need to be worked on. Another common category is a stub category, which contains very short ("stub") articles that need expansion. Wikipedia:WikiProject Stub sorting/Stub types contains a list of stubs, sorted by topic.
Container categories – categories that only contain other categories.
Set-and-topic categories – categories that are combinations of the two above types.
Universal categories – categories used to provide a complete list of articles which are otherwise normally divided into subcategories.
How else can I navigate articles?
You will most often find related articles simply by clicking the links that appear in an article. To find which articles contain links to the article you are reading, click What links here at the left-hand side of the page. For a general starting point to browsing the encyclopedia, go to Portal:Contents. Wikipedia has many lists, which serve a similar function to categories.
Can I specify categories when searching?
You can use the incategory search parameter to include only the pages in a particular category, or to exclude pages in a category as explained at Help:Searching.