A graph is a visual representation of a set series of objects where some (or all) of the units displayed are connected by links. The linking elements may take the form of lines or curves alike.
The interconnected objects are depicted by abstract mathematical units called vertices (sg vertex; ie the basic building blocks in graph formation and graph theory). 
The linking elements establishing the connection between vertices are, in turn, known as edges
Graphs are used to show clearly and distinctly how two (or more) sets of figures of measurements are related.
In graph theory, the following types of graphs are distinguished:

Example: Graph displaying revenues in the period 2007 -- 20011


For more information on graphs, you may visit the following links:
1 Graphs (Cambridge University Press)
2 Graphs and Graph Theory
3 IELTS Graphs