Thank you very much for your extensive answer!
Looks great !
But how would I save things with the relations I mentioned above, please :
User //the users
Tags //the tags itself
Tag-relations //stores the tag relations, contains references to document_id and the tag_id
Documents // some kind of document such as pdf's or the image from your example, has a relation to a user
How would I add tags to documents, while documents belong to a user ?
~Marc