Kurt Lewin (1890 - 1947) is known as the Father of Modern Social Psychology. He was a pioneer in the study of group dynamics, the study of how groups interact and the processes that take place within groups. He was also one of the first people to study Organizational Development, which focuses on finding ways to increase an organization's effectiveness.
Lewin formulated the Field Theory, which states that behavior is determined by the totality of the situation in which an individual finds himself. He came up with a formula called Lewin's Equation to illustrate the idea that Behavior is a function of the Person and his Environment (expressed as B=f(P,E).
He also came up with a framework to study the different forces that make up the totality of the situation, and called this Force Field Analysis. Lewin believed that different forces that influence a situation may help or hinder the attainment of a goal. For example, your decision to take up a course of study may be influenced by your own interests and abilities, your parents' wishes, your financial capability, your friends' opinions, and other factors.
Lewin is also credited for coining the term "Action Research," which refers to a spiral of steps that guide problem solving in an organizational or social setting. It is basically composed of a series of data gathering, planning, implementation of action, and evaluation, followed by another round of planning, implementing, and evaluation. The process is continued until the objective is met.