A Usergroup is a Container for a bunch of User, where each User can be member of several Groups.

Access permissions on objects (like pages/images) are always given to Usergroups, what makes it easy to show different content to different User.

On the other hand, a Usergroup has a set of Group Permissions. As a User can be member of several Usergroups, you can create any possible combination of Group Permissions.

FIXME add manual for Bigace 3