A "Group" is a user defined folder system for organizing and referencing your domains. For example, you could arrange your domain names by theme, creating groups such as "Games domains" and "Sports domains".
Groups can be ordered in a hierarchical structure with top level "Parent Groups", second tier "Sub-Groups" and so on. Please note, a domain name can only exist in one group at a time.
Once you have organized your domain names into groups you can create revenue reports, custom Domain Analytics reports and view/modify domains through Traffic Control by group.