Graph Theory is the mathematical study of the properties and applications of graphs. Graphs are mathematical structures used to model pairwise relations between objects. Graphs are also referred to as networks and contain a set of vertices/nodes/points connected by edges/links/lines.
Graph Theory was first introduced and studied in 1736 by Leonhard Euler who was interested in solving the Konigsberg Bridge Problem. Konigsberg was a city in Prussia, Russia with the river Pregel flowing through it creating two islands. The city and islands were connected by seven bridges. The goal of the Konigsberg Bridge Problem was to devise a walk through the city that would cross each of the 7 bridges once and only once with no doubling back, ensuring that you ended where you started.