A category page contains a list of all of the articles that have been added to it. By adding a page to a category, it allows users to search more easily for articles with something in common - and that something in common is whatever the name of the category is.
For example, on the DC Database, we put every comic in which a certain character appeared into a category, so that users can visit that category page and see everywhere else that character has appeared.
So, if you wanted to know every comic book in which The Modern Age's Batman appeared, you could look at Category:Bruce Wayne (New Earth)/Appearances, and see the (admittedly huge) list.
As you can see, categories use the "Category" namespace, which means that every category page's name begins with the prefix "Category:" followed by the actual name of the category.
We even have a category collecting every character who is a cat at Category:Cats.
A list of all of our categories can be found at Special:Categories.
How to Use Them
Adding Categories
To add an article to a category, you can use the tool bar on the right of the edit box, which contains a Category drop-box (between "Templates" and "Add features and media") to input the category you want to add.
Alternatively, you can add an article to a category by inserting a string of code into the article's text in the main edit box. That string takes the format of [[Category:Name of Category]]. For example, typing [[Category:Help]] on this page adds it to Category:Help.
Setting a Sort Key
Thanks to Wikia's "Exhibition Mode", you can view the articles within a category in an intuitive visual way with tiles, and you can sort those articles in three ways: alphabetically, by most visited, and by recently edited. These can be very convenient.
However, many people prefer to use the "Page Mode", which simply shows plain-text links in alphabetical order.
The option to switch between exhibition and page modes appears as a set of icons in the upper right of every category page, with page mode on the left and exhibition mode on the right.
If you wish to ensure that an article is sorted by a particular letter or number, rather than by the first letter of the page name, you can do that.
For example, we automatically add comics to a category for the volume they belong to, and we sort them by the issue number - which is more convenient, for obvious reasons.
You can take advantage of this by adding your sort key (the letter or number by which you want to sort the item) to the string after a pipe ("|").
For example [[Category:Cats|B]] would sort whichever page (let's say it's a page called "The Baby") into the category under the letter B.
Linking Categories
To link to a category instead of adding a page to that category, simply place a colon (":") to the string, like so: [[:Category:Comics]].
Be careful not to forget that linking to a category does not add to a page to a category, but forgetting a colon will add the page to the category and your link won't work. So remember which you want to do, and how to do it.
To present a link to a category as a piped link, so that the "Category:" prefix doesn't show, you can add your text after a pipe, ("|"). So, for example, [[:Category:Cats|Cats]] would return Cats. Alternatively, you could just say, "Here's a list of all the cats in the DCU!"
Sub-categories
Category pages can be added to other categories as sub-categories. For example, you could edit [[Category:Superheros]] and add the category [[Category:Characters]]. The Superheros category would then be a sub-category of the Characters category.
Category:Characters itself has a number of subcategories.
For Wikia's standard Category help, See Here.