The study of social phenomena identifies humans as the essential entity and attempts to describe human behaviour. Probably, the opinions an individual holds represent the most important factors behind the human behaviour. Human beings take actions according to their beliefs therefore, the investigation of the mechanism of opinion formation assumes great interest in the study of social phenomena. This mechanism depends on large number of variables and looking for a mathematical model to describe such a complex process might seem pretentious. However, many models have been proposed since the late 50's which have provided many insights into the process of opinion formation.