3D objects in a scene can be organized into hierarchies.
For information about hierarchies and transformations, see Transformations and Hierarchies [Working in 3D Space].
For information about animating transformations in hierarchies, see Animating Transformations in Hierarchies [Animation].
For more information on hierarchies specific to skeletons, see Building Skeletons [Character Animation].
Parent-child links act like constraints in many ways. If you want to have objects move together without them being in a hierarchy, you can use constraints. For more information, see Animating with Constraints [Animation].