"The trait of extraversion–introversion is a central dimension of human personality theories. The terms introversion and extraversion were first popularized by Carl Jung, Although both the popular understanding and psychological age differ from his original intent. Extraversion tends to be manifested in outgoing, talkative, energetic behavior, whereas introversion is manifested in more reserved and solitary behavior. Virtually all comprehensive models of personality include these concepts in various forms. Examples include the Big Five model, Jung's analytical psychology, Hans Eysenck's three-factor model, Raymond Cattell's 16 personality factors, the Minnesota Multiphasic Personality Inventory, and the Myers–Briggs Type Indicator.
Extroversion and introversion are typically viewed as a single continuum. Thus, to be high on one it is necessary to be low on the other. Carl Jung and the authors of the Myers–Briggs provide a different perspective and suggest that everyone has both an extraverted side and an introverted side, with one being more dominant than the other. Rather than focusing on interpersonal behavior, however, Jung defined introversion as an "attitude-type characterised by orientation in life through subjective psychic contents" (focus on one's inner psychic activity); and extraversion as "an attitude type characterised by concentration of interest on the external object", (the outside world).
In any case, people fluctuate in their behavior all the time, and even extreme introverts and extroverts do not always act according to their type."