First of all "Anonymous User" is deprecated and will be removed from book setting in the studio in our future update. This is because our publishers do not wish to to let general public to view their work before they are published, which is quite fair.
Registered users are anyone who can login and enter the studio. You might think that, well in effect anyone can register and login. So what is the difference with anonymous user? The difference is that the former can only access his or her own book. The latter was able to access anybody's book and we stopped that before KNiTO's launch.
Roles and permissions: We are going to update this too. Currently, a book owner can create "invite code" and give it to a registered user to access his or her book. Alternatively, the book owner could invite by email. In both cases, the owner, is able to assign a role such as Translator, Media manager, Editor, etc. One would naturally think that a Media Manager would only have permission to deal with images and the like. However, this is not the case in reality. Currently, no matter which role we assign to a registered user, he or she can have almost full access to a book. Therefore, it is upon users themselves to observe some restrictions.
So, we are going to change this and in our next upgrade we will be defining some standard roles, with strict permissions. Stay Tuned!