When you click on a grouped object, a dashed rectangle appears which completely contains every one of the objects grouped with the one you clicked on.
Object grouping can be hierarchical (or nested); you can create 2 groups of objects and then treat each of these groups as a single object to form another group. This nesting of groups, however, is not recommended (you will get the scanning warning 178240), as it can create confusion in QuarkXPress during diary generation.
Starting with QuarkXPress version 8 and above, grouped boxes can be used as anchored boxes.
See also: locked objects.