Object Hierachy or ‘Parenting’ is the term used to explain relationships between objects. Objects can be Parents, Siblings or Children. The structure is clearly seen in the Scene Objects panel.
Children act relatively to thier parents position, rotation and size. They also adopt their parents visibility and lock settings. Objects settings only affect lower hierachical objects. If you lock a child it will not lock its parent.
This is a powerful feature than can be used for different purposes. You could use parenting as a way to organise your projects objects or you may use parenting with stacked controllers to rotate and translate from different axes.
To create the hierachy simply drag and drop an object in the Objects Tree onto another object to create a child.
Parenting is not to be confused with Selection Groups.