Graph

Graph is a finite set of points { $P_1, P_2, \dots, P_n$}, called vertices or nodes, together with a finite collection of ordered pair ($P_i, P_j$) called edges.

As is shown in the following figure the points in the set are called vertices and the relation or ordered pairs are called edges.

Figure 1

A simpler example of a graph is shown in the following figure:

Figure 2

In this graph $P_1, P_2, P_3$ are vertices, and ($P_1, P_3$) and ($P_2, P_3$) are edges.