Go through my previous post about What are Trees if you haven’t already read it to completely understand the binary trees.
In general, tree nodes can have any number of children. In a binary tree, the nodes have no more than two children. Figure 1 has examples of binary trees. Figure 2 isn’t a binary tree because several of its nodes have three children. A node in a binary tree isn’t required to have two children. For example, node b in Figure 1 has only one child. Of course, the leaves of a binary tree have no children.
Every tree has a height, which is defined as the maximum number of nodes that must be visited on any path from the root to a leaf. For instance, the tree in Figure 1 has a height of 3. In turn, the depth of a node is the number of nodes along the path from the root to that node. For instance, node c in Figure 1 has a depth of 1. Also go through How to Find the height of a Binary Tree.