Help:Introduction to Categories
Categories are an excellent way to arrange various articles in a way which makes them accessible and well-arranged.
To add an article to a category, simply put [[Category:Category name]] in an article. (Convention dictates that this be at the bottom of the article, but it could in princible be anywhere). This creates the category if it did not already exist. However, since the category is empty of any descriptive text in the article, it will still appear as a red link until there is text in the category.
To arrange a subcategory of a category, simply place the subcategory in the parent category as you would place an article in a category. The subcategory will automatically be placed in the parent category.
To link to a category without placing the article in this category, prepend a colon to the link, as so: [[:Category:Category name]]