All Web Designer objects exist in an object ownership hierarchy, where larger objects own smaller objects. Container objects however, are special because containing other objects is their main purpose. For example, column controls are container objects. Their main purpose is to contain columns.
Container objects make it easier to organize and maintain the layout of controls in a window. For example, you can draw a group border around a group of controls. You can also specify that a group object has vertical orientation; this causes the objects within the group (for example, text boxes with prompts) to appear neatly underneath each other.
Web Designer automatically implements container objects. They include column controls containing controls for each column in a base table. Group objects are specified by the developer. They can containing any selection of objects, and are often to make layout of user-defined controls easier.
Note that group controls are also container objects, but they have now been replaced by container controls. Group controls will be phased out eventually.