Try thinking of it this way...
- A group is a way to organize users.
- A category is a way to organize content (such as a wiki page or blog).
Typically, I use groups to organize users:
- by permission
For example, members of the Editors group might have the tiki_p_edit permission and can edit wiki pages. But members of the Registered group may not.
- by content type
For example, members of the Group A Users group might be able to view only content that is in the Group A Pages category.
Both groups and categories are closely related to Tiki's permissions. I recommend reviewing the docs for details.