A group is a subset of objects within a model or scene that move and behave as a single entity, yet can still be split apart (ungrouped), if necessary. Most complicated models are constructed from several less complex parts that need to maintain the same spacing and orientation. Grouping provides a way of locking the relative positions of the objects without joining them permanently.
You can group parts to animate the group together. Parts do not have to be near each other to be grouped.
To assign parts to groups: