What types of visibility/privacy settings are available when configuring a group?

There are multiple group configurations, including:

Secret: Users can only join by invitation and the group itself and its content are hidden from nonmembers.
Public: Users may join without approval and all content added in this group will be visible to all community members and anonymous users.
Open: Users may join without approval and all content added in this group will be visible to all community members.
Closed: Users can only join by invitation and the content in the group is hidden from nonmembers.