Personality is the sum total of how an individual reacts and interacts with others. It is influenced by heredity, environment, and situation. There are several theories that describe personality types and traits, including the Myers-Briggs Type Indicator, Big Five model, and locus of control. The Myers-Briggs Type Indicator classifies people into 16 types based on four characteristics, while the Big Five model describes personality along five dimensions such as extroversion and agreeableness. Freud's psychoanalytic theory proposes that personality has three parts - the id, ego, and superego - which develop at different stages from birth to adulthood.