A Group as used in Organic Groups is a collection of Content and/or People. Groups can be what you find on groups.drupal.org or Facebook. But do not be mistaken, this is the simplest form of a Group. It is important that you think of Groups as Defined within the English language and not within the confines of pre-existing notions and limitations of current groups.
Social Networking Groups
Organic Groups can be configured to pre-existing social networking concepts such as those that are found on Facebook, Tagged, MySpace and even Groups.Drupal.org (g.d.o.)
Social Networking Groups can be created and maintained by the users of the site allowing the members of your site to take control over their social experience and the content they desire or find appropriate to the topics they desire to explore.
Groups essentially removes the rigid boundaries of current "forum" based sites, and allow the users of the site to define their own topics and the scope in which these topic contain. Groups can also be self moderated allowing Group moderators the ability to control the audience of their content. Thus Groups can be open to all, moderated approval, or invite only and the ability to maintain their user base is distributed to group owners.
Organic Groups allow for very unique content management custom solutions. Content of the "Group Content Type" has the unique ability to belong to multiple Groups at once. This reduces duplication and increases content accessibility. Utilizing users to adopt content allows users to create their own libraries of content without having to create duplication. Content is thus associated with the user and can be maintained by the user, and group maintainers can then manage content in ways that help their audience.
Groups provide unique opportunities to create user relationships based on Nodes and/or Content, rather than the typical friendship modules. This does not diminish the friendship based relationship model, but provides a unique model based on structured relationships. Since Groups are based around Nodes, Administration of the user relationships can be assigned to a moderator(s). This structural approach can meet unique needs for distributed moderation while also providing the option for freedom of user created relationships.
The flexibility of groups is broad. People have used groups in a wide variety of sites. Often times the word "Group" is never seen by the end users.