Tree terminology: If n is a node, and p is the node connected to it in the preceding level, then we say that node p is the "parent" of node n and node n is a "child" of node p. The root node does not have a parent because there are no nodes above it.
In pictures the nodes are represented as circles and the connections are drawn as edges that connect the circles. See below.
A tree is said to be a "Binary Tree" if each node is connected to no more than two nodes in the level below it. Each node has at most two children. In addition the nodes on a level are ordered from left to right. A node can have a "Left Child" or a "Right Child", or both a left and a right child, or no children at all. A node without any children is said to be a "leaf".
In the binary tree above, the node containing 16 has a left child, but has no right child. And the node containing 14 has a right child, but has no left child. The nodes containing 13, 2, and 3 are leaves because they have no children.