Humor phenomenon can be defined in terms of expression and perception of funny stimuli, such as jokes. Various studies of humor phenomenon are reviewd, and types of humor are categorized in this paper. First, theories and hypotheses on humor are briefly reviewd; the theories discussed are superiority theory, incogruity theory, theories by Freud and Berlyne, theoretical thinking in humanistic psychology, and stress-moderating hypothesis. Each theory along with related studies is examined and characterized in terms of the humor phenomenon framework. It is suggested that the past research in the field has focused mainly on three subjects: 1) motivation behind expression of humorous stimuli, 2) cognitive processes for perceiving such stimuli, and 3) psychological consequences of humor perception. Finally, a new classification scheme for humor in terms of motivation is proposed, with categories of playful, aggressive and support-relief humor, for clear and coherent understanding.