An early form of personality type indicator theory was the Four Temperaments system of Galen, based on the four humours model of Hippocrates; an extended five temperaments system based on the classical theory was published in 1958. One example of personality types is Type A and Type B personality theory.

The Big 5 personality traits are extraversion (also often spelled extroversion), agreeableness, openness, conscientiousness, and neuroticism. Extraversion is sociability, agreeableness is kindness, openness is creativity and intrigue, conscientiousness is thoughtfulness, and neuroticism often involves sadness or emotional instability.

Each person is a unique combination of four personality types. Over the centuries, these basic categories have gone by several names and designations, but for our purposes, they're known as the director, the socializer, the thinker, and the supporter. As shorthand, though, we refer to those types of personality as A, B, C, and D, respectively.