http://www.humanmetrics.com/cgi-win/jtypes2.asp

The Basics of Jung's TypologyJung called Extraversion-Introversion preference general attitude, since it reflects an individual’s attitude toward the external world distinguished by the “direction of general interest” [Jung, 1971]: the extravert maintains affinity for, and sources energy from the outer world, whereas the introvert is the other way around – their general interest is directed toward their inner world, which is the source of their energy.

As mentioned above, Jung introduced a pair of judging functions -thinking and feeling - and a pair of perception functions – sensing (or “sensation”), and intuition.

Sensing-Intuition preference represents the method by which one perceives information: Sensing means an individual mainly relies on concrete, actual information - “in so far as objects release sensations, they matter” [1], whereas Intuition means a person relies upon their conception about things based on their understanding of the world. Thinking-Feeling preference indicates the way an individual processes information. Thinking preference means an individual makes decisions based on logical reasoning, and is less affected by feelings and emotions. Feeling preference means that an individual's base for decisions is mainly feelings and emotions.

Jung introduced the idea of hierarchy and direction of psychological functions. According to Jung, one of the psychological functions - a function from either judging or perception pair – would be primary (also called dominant). In other words, one pole of the poles of the two dichotomies (Sensing-Feeling and Thinking-Feeling) dominates over the rest of the poles. The Extraversion-Introversion preference sets the direction of the dominant function: the direction points to the source of energy that feeds it – i.e. to the outer world for extraverts and to the inner world for introverts.

Jung suggested that a function from the other pair would be secondary (also called auxiliary) but still be “a determining factor” [Jung, 1971]. I.e. if Intuition is dominant, then the auxiliary one is either Thinking or Feeling. If Sensing is dominant, then the auxiliary one can also be either Thinking or Feeling. However, if Thinking is dominant, then the auxiliary one is either Sensing or Intuition, and if Feeling is dominant then the auxiliary one is either Sensing or Intuition. In other words, the auxiliary function never belongs to the same dichotomy.

Jung called feeling and thinking types “rational” because they are characterized by the dominance of judging functions that provide reasoning rationale (be it thinking or feeling). “Rational” or Judging preference results in thinking, feelings, response and behaviour that consciously operate in line with certain rules, principles or norms. People with dominant "rational" or judging preference perceive the world as an ordered structure that follows a set of rules.

He called sensing and intuitive types “irrational” because they are characterized by dominance of the functions of perception (either sensing or intuition), and therefore their “commissions and omissions are based not upon reasoned judgment but upon the absolute intensity of perception” [Jung, 1971]. “Irrational” or Perceiving preference operates with opportunities, i.e. with a range of possible outcomes that result from assumed premises or from sensations, mostly driven by the unconscious processes. People with dominant "irrational" or Perceiving preference thinking see the world as a structure that can take various forms and outcomes. It is possible to determine, either by observation or by asking certain questions, preference of Judging vs. Perceiving and the strength thereof in a person.