People use categories to make it easier for other people on a wiki to find articles. But it's sometimes hard to do it right.
When you're editng you find in the right rail the section "Categories". Tick in the category name and click on enter. After that you have to publish to add them to the page. Another way is adding them on the page, go to the bottom and you'll find a bar where you can do.
Add Categories to Categories
To make a good structure of your categories, you have to add first the category to a page. Then go to that category, click on edit and add also there a category. Now you can go to that category and you see the category shows the category you add to the category. You can do the same with that category if you want.
Make sure you add pages to the right categories, otherwise it'll confuse people. It's also important to make obvious named categories so people can add them easily. Also it's a good thing when the categories doesn't have long names.
Categorize Page on Other Name
If you want to categorize your page on another name/character, hover over the category name in the editor and click on the edit icon. Now you see a screen where you can type it and save.
Delete Category of Page
If you want to delete a category on a page, you simply have to hover over the category name in the editor and click on the delete icon.
Edit Category Pages
On category pages you can add text, but also things like pictures. It's nice to do this on the biggest and most important categories. You can write something about the category or explain something of it.