Yes, categories can be used just like tags, just named differently, and they are independent of how your content is organized. You may want to have a look at this post about how to remove the “tag” from the url and get nice and clean ones by using routes How to cleanup/remove the (tag:) from url.
Of course, if you prefer to have folders for your categories, that would be another option to organize your content and more “putting content into categories”. There are many ways to achieve results in Kirby 
It wouldn’t make sense if you were to assign more than one category to a post, though.