What is a Binary Tree?

A binary tree embodies a finite set of data items that is either empty or partitioned into three disjoint subsets. The first part contains a single data item referred to as the root of the binary tree, other two data items are left and right subtrees. These data items is referred to as nodes of the binary tree.

From this figure we can portray the following:
A - Root node of the binary tree
A - Parent node of the nodes B & C
B & C - Children nodes of A and these nodes are in a level one more than the root node A
C - Right child node of A
B - Left child node of A
D & E - These node are without a child, so they are referred to as leafs or external nodes
B & C - These nodes have a child each, so they are referred to as internal nodes

Depth of this binary tree - The longest path from the root node A to any leaf node, for example D node. Therefore, the depth of this binary tree is two.
Indegree of a node - The number of edges merging into a node. For example, indegree of the node B is one i.e. one edge merges.
Outdegree of a node - The number of edges coming out from a node. For example, outdegree of the node A is two i.e. two edges comes out of this root node.

Concerning the binary trees, do refer to the following blog entries:
- Exercise-Binary Tree Construction
- Application of a Binary Tree