Groups are gatherings of members, posts, and any other user-generated content form the group members. In summary, it allows you create a micro-community within the community. This is useful for maintaining privacy in communications and postings between group members.
Any member can create a group. When a member creates a group, this person automatically becomes the group administrator.
Group administrators can also approve requests to join the private group, invite friends to congregate within a hidden group, establish communications among members, etc.