In the docs:
http://doc.silverstripe.com/doku.php?id=datamodel
it is explained how to add a different categories to an article.

How do I add categories to an article? Do I need to make a pagetype category and then add diffrent pages for each categorie or can this be done when selecting an article and add a category on the fly? Is it possible to add more than 1 category to 1 article?

Make a Category as DataModel only, it need not to be Page. If you make many_many relation between Article and Category, then you may have more Categories for one Article.

SS supports all main database relations: has_one (1:1), has_many (1:N), many_many (N:N).

xmedeko,

Where do I administer the categories? In the document it shows how to add the categories but what CMSfield type do I need to use?

Thx