Categories are used to organize the pages of a wiki. Help:Category describes how to add or remove a category. This page will help you understand what kinds of categories to add and how to organize them.
Regular pages versus category pages Edit
When you add a category to a regular page, you are saying "this page is a part of this category". When you add a category to a category page, you are saying "everything in this category is also a part of this other category I just added". This can cover more pages than you want and may not be correct in all cases. It doesn't mean "some of these categories are also in this new category". It means all of these categories are in the new category. So, in general, it is safer to add a category to a regular page than it is to a category page.
For example, a Golden Retriever is a dog, and dogs are canines. If you were to put [[Category:Dogs]] on the Canines category page, you would be saying that all canines are dogs. This would not be correct because canines also include wolves, jackals and coyotes.
However, you could put [[Category:Canines]] on the Dogs category page, because dogs are a type of canine.
On the page for a terrier, you would put [[Category:Dogs]] on it. You could also put [[Category:Canines]] on its page, but it may be simpler to just use the Dogs category alone.
Another example from some Disney movies involves rats. There are common rats in films like Lady and the Tramp, Oliver & Company and Tinker Bell and the Lost Treasure. They are a lot different than Professor Padraic Ratigan from The Great Mouse Detective and Remy from Ratatouille.
While Ratigan and the rats from the first four movies might be killers, it would be wrong to put [[Category:Killers]] on the Rats category page because Remy is not a killer. But if you did that, that's what you would be saying: "Every character in the Rats category is a killer".
Another example is a category for Pets. If you go to the page for Category:Pets and edit the page to add Category:Dogs, it means every pet is a dog. That's not the case. Not everyone has a dog as their pet. Some have cats, ferrets, hamsters, gerbils, fish, tarantulas or any number of different types of animals as their pets.
It is safer and more accurate to put categories on individual pages than to put a category on a category.
Choosing categories Edit
When a wiki is getting started, categories can be chosen for the most-likely subjects and may wind up not having very many pages in them. After the wiki is established, users need to take care not to create categories unless they would actually be needed. In this case, being needed refers to how many pages will be in that category. After a few years, the members of a wiki should have a pretty good idea of what kinds of subjects, and therefore categories, a wiki will cover.
Using the Golden Retriever again as an example, if there were only one or two of them on a wiki, then having a "Golden Retrievers" category probably wouldn't be necessary and could instead by covered by the "Dogs" category.
Another factor that needs to be considered would be "is this category name too long or too specific?" You could have a "Golden Retrievers with red body fur and black leg fur" category. But unless there were a large number of pages about Golden Retrievers that have that specific fur pattern, it would not be necessary or could be better served with a more-general category like "Golden Retrievers with unusual fur" or even "Dogs with unusual fur".
Overly-descriptive category names Edit
The longer a category name is, the more difficult it is to use. When typing the category, the wiki will automatically suggest existing ones that match what you've typed so far. If the auto-suggest doesn't work, displays several choices, or there are more choices than it can display, a person may have to type the category by hand. In these cases, spelling mistakes can lead to new categories being created by accident.
A category name like "Golden Retrievers with red face fur, fair body fur and black leg fur" is a lot for a person to type and takes up a lot of room at the bottom of the page when the categories are displayed. A shorter category name like "Golden Retrievers with unusual fur" would be better. A few more details about what the unusual fur is could be put on the category page itself.
An overly-descriptive category name may also be of interest to only one person or a small handful of people. When picking a name for a category, ask yourself, "Is this something that a lot of people are going to be interested in, or just me?" If the answer is "just me", you probably shouldn't add that category. Look for an existing category to use instead.
Long category page descriptions Edit
When creating the page for a category, the information should be kept short as a general description of the category. In general, no more than one or two sentences.
Category pages should not be used to replace a regular page. If a category page has a lengthy discussion, then that information needs to be moved to a regular page. Remember, categories are used to organize a wiki.
Duplicate categories Edit
If there are two categories that are saying pretty much the same thing, see if one of them could be used in place of the other. For example, if you had "Poodles with very pale brown fur" and "Poodles with beige fur", the beige category could be used instead because beige is that kind of color and it is a shorter category name.
In some cases, users may put both categories on a page because they think both fit or because they're relying on the auto-completion feature of the wiki and blindly accept both categories.
Another source of duplicate categories comes from how wikis treat capitalization. A category of "Small Dogs" and one of "Small dogs" looks the same, but because the first one has a capital D and the second one has a small d, they are separate. Administrators should set a standard on capitalization for categories and update pages to consolidate them into a single category.
Excessive categories Edit
Too many categories present too many choices for people. If there are more than a dozen categories on a page, consider removing some or consolidating them into fewer categories. Categories that look like they are of only of interest to a few specific people are good candidates for being replaced by general category.
On short and stub pages, avoid having more categories than actual content on the page. A lot of the time, what's in the categories could just as easily be put on the page itself, which helps fill it out. For example, instead of using categories to label the character as "Arrogant" and "Hot-headed", provide information in the main part of the page to show how they are arrogant and what makes them hot-headed.
On wiki that have Achievement Badges enabled, it is tempting to add a category just to earn a badge. This can lead to frivolous, overly-descriptive or even just plain incorrect categories being added. A category should not be added just to add a category and get a badge. It should be added when the category will help better organize the page and is a subject that other people would be interested in.
By trimming down the number of categories or setting a firm limit on how many can be added helps avoid situations where there are dozens or hundreds of categories and almost no information on the rest of the page.
Category spamming Edit
Category spamming occurs when a person spends a majority of their time only adding categories to pages. This can be either because it is easy to do by clicking on the "Add category" button at the bottom of the page, or it can happen as a result of that person trying to earn badges in the Category track.
Adding a few categories, especially to earn those badges, are okay, but take care not to go overboard. If you are adding a category just to add a category, very often you are just adding clutter to the page rather than helping to organize the page. This can lead to the excessive categories situation described above.
Administrators should decide if category spamming is enough of a problem on their wiki to move it from being a nuisance to being a blockable offense. If it is, that should be added to their block policy or code of conduct.
Administrators may want to review the list of categories at Special:Categories to see if there are any with only a few pages in them and determine if those categories could be consolidated together or replaced by a general-purpose category.