A wiki category is a way of grouping related articles and other related categories. Categories can contain articles and other categories, nested as deeply as needed.
Category structure example
Here is a sample diagram of some top-level categories in this wiki:
This example shows the top-level category, which is named top level. It's just an arbitrary name chosen for the top of this wiki's hierarchy (and it can be changed). This category contains other categories, including projects, terminology, and help. Each of those categories, in turn, contains other categories or articles.
Belonging to multiple categories
An article or category can belong to as many categories as are appropriate. In the preceding example, the DOM terminology category belongs both to the terminology and the DOM project categories.
How to browse categories
At the bottom of each article, click the name of a category to browse within categories. (For example, at the bottom of this article, click Help to see what else is in the Help category, and from there you can also browse up and down the category hierarchy.)