One of the main concepts in modelling, as in life, is the concept of an ever-present hierarchy.
Hierarchy is defined as the order of importance for a group of objects, let me try to explain.
Imagine that these three poles are a family, the biggest being the most important, this is a group. All of the objects will move exactly the same way at exactly the same time.
Now, the tallest pole is unselected. Think of the tall pole as the father of the family leading the way, the parent if you will.
Now, only one of the objects are selected, the smallest pole of the bunch. Think of this pole as the child.
This rather odd family of models can also be altered by some other factors, like constraints or deformities. What these two do is alter the way in which a model moves or can be moved. For example, if you apply a bend deformity to an object, the object would bend around the ______. Constraints and deformities can also be used with parents and childs. Giving a parent a constraint or deformity can alter all of the grouped models, whilst adding a constraint or deformity to a child will only effect that object, nothing else.