A hierarchy is a list of things that are linked together in a certain order.
A family tree is used as an analogy to help describe the parts of the hierarchy. However, the hierarchy tree traditionally hangs upside down, to make it easier to read and to use. At the top of the tree is the root, and all things are attached to it. Every object in the hierarchy is called a node. The connections between the nodes are called links. A node linked to another is called a child, and the node the child is linked to is called a parent. A parent can have multiple children, but in this tree each child can have only one parent. The nodes that have no children are called leaves, because they're at the ends of the tree.
Each node can trace its lineage up through this upside-down tree, back though parents to the root. If you choose any parent, then you can call its collection of children a branch of the tree.