There are 3 different types of groups that you can create in your community: Public, Private, and Secret.
1. Public groups are accessible for all community members, and anyone from the community can view the content before joining. Joining public group does not require any approval. Public groups are also visible on the "all groups" page.
2. Private groups on the other hand are only accessible to its group members. Users can see the posted content in the group before joining. Joining private group requires approval from group admin, group moderator or the community admin. Private groups are also visible on the "all groups" page.
3. Secret groups are like private groups, with this difference that they are not listed on "all groups" page so users can not discover them unless they are invited to the group. The only way to join a secrete group is the invitation from group admin, group moderator or the community admin.