<p><em>Taken from Wikipedia, the free encyclopedia:</em></p>

<p>Informally, a graph is a set of objects called <em>vertices</em>
(or <em>nodes</em>) connected by links called <em>edges</em>
(or <em>arcs</em>), which can also have associated directions.
Typically, a graph is depicted as a set of dots (i.e., vertices)
connected by lines (i.e., edges), with an arrowhead on a line
representing a directed edge.</p>

<p>Such graphs can used to represent and analyze a variety of
systems and problems, including colorability problems, shortest
path algorithms and spanning trees.</p>
