Categories are a method to group like pages. Visitors can then visit the category links at a bottom of each page. It is similar to the See Also section of a page but with a much looser tie between each page. For example there is a category for all Category:Containers.
Linking to Categories
There is a special notation to link to a category page. You use a colon at the beginning of the category tag. For example if you want to link to the category of House Addons you do it like so: [[:Category:House Addons]]
Determining Appropriate Categories
Each category has an explanation of what pages belong. For example the Category:Mining includes all items/subjects related to the Mining skill. It won't always be 100% clear whether a category belongs or not. Don't be afraid to add a questionable category, it can always be removed later.
Category tags should always be the very last items on a page in the footer. They should appear in alphabetical order, each on its own line. For example here is the appropriate tags and layout for the Elven Dresser: