Trees are useful for modeling data that has a hierarchical relationship which moves in the direction from parent to child. No child node will have more than one parent.

To recap some terms:

• root: A node which has no parent. One per tree.
• parent: A node which references other nodes.
• child: Nodes referenced by other nodes.
• sibling: Nodes which have the same parent.
• leaf: Nodes which have no children.
• level: The height or depth of the tree. Root nodes are at level 1, their children are at level 2, and so on.