Help:Category
You can place a page in a category by adding [[Category:foobar]] to the article's text (where foobar is the name of the category).
For example to add an article called Albert Einstein to the category "people" you would edit the article and add "[[Category:People]]".
Setting sort keys
If you want the article, in the list of articles in the category, to be positioned alphabetically by something else than its name you should use the following link form: [[Category:foobar|sort key]] (where foobar is the category name and sort key is the value to sort by).
For example to add an article called Albert Einstein to the the category "people" and have the article sorted by "Einstein, Albert" you would type "[[Category:People|Einstein, Albert]]".
Linking to a category
If you want to link to a category without the current article being added to it you should use the following link form [[:Category:foobar]] (where foobar is the category name), note the extra : before Category.
Sub-categories
Category pages can be added to other categories and will be automatically recognized as sub-categories. For example you could edit [[Category:Soccer]] and add the link [[Category:Sport]]. The Soccer category would then be a sub-category of the Sport category.
Viewing a list of categories
You can see currently used categories for each project with Special:Categories, for Wikipedia en:Special:Categories.