The Groups and categories features are some of what makes tikiwiki stand out from the other wiki/cms software out there.
"Groups" are sets of users "Categories" are sets of content (articles, blog posts, wiki pages, etc.) 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.