ABSTRACT
Graphs are a very important theme in mathematics and computer science, and provide an important way to understand very complex systems such as the networks that pervade biology, in terms of the various entities that exist and their relationships. Graphs are essentially the mathematical formalisms that are used to study networks, but the two terms are invariably used interchangeably. Graph theory has a very interesting historical origin, and is a tribute to the intellectual genius of Leonhard Euler. Every graph needs to be described by a set of nodes/vertices and a set of edges. Graphs can be classified into different types, based on the nature of the connections, the strengths of the connections and so on. This chapter reviews some of these basic concepts. Simple graphs are graphs without self-loops or multi-edges: a self-loop involves an edge from a node to itself; a multi-edge means that there is more than one edge between a pair of nodes in a graph.
