Categories are used to categorize and sort pages in the wiki. They are required to keep track of a very large amount of pages, articles and files that would otherwise be very hard to find. Categories are used by both regular viewers and editors. Viewers can use it as a complement to the search function, as it provides a clear tree-like structure that allows a viewer to navigate the thousands of pages located in the wiki.
For an editor, categories provide some additional functions as well. By knowing how categories work and navigating them, editors can see if a specific file exists in the wiki or if they have to upload it, if a topic is covered or needs an article, and where links should point to. Categories are thus very important to include when creating a page.
A page is categorized by simply linking the article to the category with a regular wiki link:
[[Category:Name of Category]]
Adding a category like this automatically places the article in that category, and will include a link to that category at the very bottom of the article. Although the category link can be placed anywhere in the article, it should always be placed at the very bottom of an article so that it's easy for other editors to find if they need to change a category.
If you wish to provide a link to a category without placing the article itself there, put a colon before it:
[[:Category:Name of Category]]
This will create a regular link to the category page in the text that can be clicked. Using this format will only link to the category and will not categorize the page.
Not only articles and files can be categorized, but categories themselves. This allows for the creation of tree-like structures which viewers can use to browse and navigate between categories.
Categories and other pages can both be placed in the same category. However, categories will be sorted by themselves at the very top of the page, which all other pages (normal articles as well as images) will be sorted undearneath them.
Though categories function like regular pages in a lot of ways, they cannot be moved. This is because moving an article would not actually recategorize any of the pages within it; they would still appear to be in the same category. All that would happen would be that the text contained on the category page itself would be moved.
Because of this, categories that are for some reason incorrectly named can only be fixed by recategorizing every individual page within it and then creating a new category page with the correct name. The old category should then be deleted, which a sysop should be notified of if you can't delete it yourself.
- http://en.wikipedia.org/wiki/Help:Category A more exhaustive look at categories in general over at Wikipedia.