What are categories?
Categories allow articles to be placed in one or more groups, and allow those groups to be further categorized.
When an article belongs to a category, it will contain a special link to a page that describes the category. Similarly, when a sub-category belongs to a parent category, it will contain a special link to the parent category's page.
Each category page contains an introduction that can be edited like an article, and an automatically generated list of links to sub-categories and articles that belong to the category.
Categories do not form a strict hierarchy or tree of categories, since each article can appear in more than one category, and each category can appear in more than one parent category. This allows multiple categorization schemes to co-exist simultaneously. It is possible to construct loops in the category space, but this is discouraged.
What is the purpose of categories?
There are two main ways to use categories: lists and topics. When starting a subcategory, making an early decision about whether it is a list or a topic will reduce later renames, recategorizations, and discussions. The category page can be used to tell others whether it is a list or a topic, and to link to a main list or main article. See meta:Categorization requirements for the original purpose of the feature, and Wikipedia:Categorization for current usage and guidelines for assignment.
What categories already exist?
The best way to find out what exists is to browse. Either start at Wikipedia:Browse or start at Category:Articles, which shows the "top" level categories, to which all other categories should be connected. A list of all categories can be found at Special:Categories – there are many thousands listed but you can jump through them using the search box. Cat Scan, also at the Toolserver, can be used to find subcategories.
Wikipedia:Category schemes shows designs that predate the category feature. Some high-level categories can be found at Wikipedia:Categorization#Current projects.
As well as the standard links within articles themselves, every page has a link (typically at the side) called What links here.
What is the difference between a list and a category?
Grouping articles into a category is not the same as making a list of articles. To make a list of articles, you edit the list directly; but to place articles into a category, you edit each article and insert a category tag by placing [[Category:category_name]] in the body of the text. This automatically adds those articles as a list on the category's page. Despite the difference in how they are maintained, it is sometimes convenient to think of a category as a list of articles.
While an article may be in multiple lists, the goal is that browsing downwards from a list parent category, e.g. Category:People, should only arrive at articles that are about people, e.g. John Lennon, and not related articles e.g. 251 Menlove Avenue.
There are some natural hierarchies of lists. One example is the scientific classification of organisms, which would only place an article in one category. Other systems use multiple listings, e.g. Wikipedia could be in Category:Wikis and Category:Encyclopedias.
List categories are likely to also be subcategories of topic categories.
Maintaining and tracking categories often requires more effort than a simple list. If you have a category that has vague criteria or that adds and removes members frequently, then maintaining a simple list is often more appropriate.
Further information on this topic can be found at Wikipedia:Categories, lists, and navigation templates.
Are "List of ..." articles still needed?
There are many articles in Wikipedia called "List of ...", e.g. "List of assassins".
Lists are still useful for showing "missing" articles.
See also: Wikipedia:Categories, lists, and navigation templates and Wikipedia talk:Categorization/Archive 1#Lists v. categories.
Are topic and hierarchy info-boxes still needed?
Many articles have "info-boxes"—side-boxes or footers linking to other articles and lists, sometimes using Wikipedia:Templates. Some of these are listed at Wikipedia:Navigation templates.
While some info-boxes are made redundant, others have useful grouping of the cross-references that cannot be done with categories.
See also: Wikipedia:Categories, lists, and navigation templates, Wikipedia:Article series, Wikipedia:Incumbent series.
Can pages be in more than one category?
Yes, it is expected that most pages will be members of more than one category.
Similarly, each category can be a member of more than one parent category.
What are "maintenance categories"?
Some categories exist to aid maintenance of the project, for example, template categories and redirect categories. Maintenance categories are often added by templates,
, rather than by the "wikilink" structure,
[[...]]. Benefits of template usage include the ability to populate more than one category with a single template, inclusion of text to explain the rationale of a given categorization(s), the ability to readily include/exclude certain pages or types of pages, and so on. Contributors may monitor these maintenance categories for many reasons to include compliance with policies and guidelines that pertain to various conventions, such as page naming conventions. Maintenance categories are usually "hidden" from view and can be seen only by registered users who have set their preferences to see hidden categories, which is easy to do:
- Go to Preferences → Appearance
- Scroll down to Advanced options
- Check the box Show hidden categories
- Click Save
- It is a good idea to purge your browser cache to ensure that you can see hidden categories after setting your preferences.
Templates used to populate redirect categories are indexed functionally and alphanumerically at Wikipedia:Template messages/Redirect pages.