Hierarchies describe the relationship between objects, usually using a combination of parent-child and tree analogies, as you do with a family tree. Objects can be associated to each other in a hierarchy for a number of reasons, such as to make manipulation easier, to propagate applied properties, or to animate children in relation to a parent. For example, the parent-child relationship means that any properties applied to the parent (in branch mode) also affect the child.
In a hierarchy there is a parent, its children, its grandchildren, and so on.
A root is a node at the base of either a branch or the entire tree.
A tree is the whole hierarchy of nodes stemming from a common root.
A branch is a subtree consisting of a node and all its descendants.
