Groups are sets of users you can define and award any level of permissions Categories are sets of content (including articles, blog posts, wiki pages, etc.) that can be controlled or accessed by any or all groups.
Both Groups and Categories are fully customizable in their permission settings.
Using groups and categories, you can create your own wiki hotel where any number of groups can have private "rooms" (sets of pages) that only they (and the site administrators of course) can access.
Each group can have a group home page that they are automatically taken to when they log into the site.