A project group is a set of projects grouped together for administrative purposes. Project groups enable administrative actions to affect multiple projects and users simultaneously.
You can define the basis upon which the project groups are created. You can elect to group projects by technologies, by departments, by clients or by any other criteria that your organization may require. Regardless of how you set up your project groups, any given project on your site may belong to more than one project group.
The access to project groups is controlled through the permissions granted via a role.
Project groups make permission administration easier, whereas, Categories are considered useful for classifying projects.