• It is a collection of nodes.
• There is a Parent-Child relation for nodes.
• It stores data by value rather than position.
• There is only one root. (starting node)
• Each node has several children.  (nonlinear structure)
• Each node has unique parent except the root.
• There is no cycle.
• It is defined recursively.  (subtree)
• Leaf nodes have no children.
• A path between a parent node P and any node N in its subtree

is a sequence of nodes P = X₀ , X₁, ..., X_k = N, where k is the length of the path.
• The length of a path between the root and any node of the tree is called the level of a node.
• The depth of a tree is the maximum level of any node in the tree.

Complete Tree:
      Fill nodes level by level and from left to right.

Binary Search Tree:
      each node in left subtree < the nodes in right subtree