Terminology
Trees are used to represent relationships: items in a tree are referred to as nodes and the lines connecting the nodes that express the hierarchical relationship are referred to as edges.
The edges in a tree are directed.
Trees are hierarchical which means that a parent-child relationship exist between the nodes in the tree.
Each node has at most one parent. The node with no parents is the root node.
The nodes that have no successors (no children nodes) are known as leaf nodes.