Complicated relations between different objects/phenomena can be visualized with graphs.
a graph G is defined as an ordered pair of sets G = (V,E). where E is an ordered pair {a, b}, a,b ∈V. e.g. constitutes a pair of element in V. 

The elements in V are called nodes or vertices.

The elements in are the arcs or edges of the graph. The arc e = {a,b} connects the nodes a and b or it is incident with a and b. a and b are then called end-points to the arc ab. The nodes a and b are adjacent if  ab is an arc in the graph.

Arcs starting and ending in the same node are called loops.  

aan arc not incident with a node is called isolated.

Only one arc is allowed to run between two nodes.