As Jung generally follows the method of presenting historical evidence of an idea and then correlating this to a new theory while letting the reader do most of the actual correlating, this book is no different. But, unlike other works of his, I do not believe the correlations he uses to be as powerful or all that helpful. With other works, such as Symbols of Transformation, the relationship between the older method and the theory of the unconscious that he is explaining is rather evident, so the reader has a smoking gun to work with. Also in Symbols of Transformation, Jung begins the piece with his own work so the reader is given some interpretative tools to aid them in this process of correlation. However, in Psychological Types, I get the distinct impression that Jung is simply enumerating other older theories of personology without actually using them to support his theory. He seems to be only noting that the field of personology exists so that he can introduce his own theory. After much historical material is presented, ranging from Greek thought, Schiller, poetry to William James, Jung introduces his landmark theory of personality.
Jung begins his theory of personality by first introduction the categories of introversion and extroversion. These categories represent the direction in which the individual primarily invests his/her libidinal energy, called "attitude types." If the energy is invested in the individual itself and withdrawn from the external world so that the world loses the power to influence him'/her, they are considered introverted. If the energy is invested into the outward world, they are considered extroverted. In the first case, the individual attempts to gain a monopoly on their libido while in the former, the individual gains power through the multiplicity of their libidinal investments. Jung writes that the weakness of the introvert is that such an emphasis on being an individual can lead to a perverse attachment to the ego in such a way that denies its reliance upon others, and in its worse state the introvert can laps into solipsism. In the case of the extrovert, their weakness is the risk diversifying their energy too much and even the risk of losing themselves in the objects around them. Jung also warns that fundamental impulses of individuality such as thoughts, feelings, needs and wishes can pass by unacknowledged, paving the way for neuroses.
Next, Jung breaks down the individual further into four primary functions; thinking, feeling, sensing and intuition. Thinking and Feeling Jung classifies as "rational" functions. I think of this in the Greek sense of the term "ratio" which is rational's root, along with the root of ratio. In a ratio, one proportion is found to be equal to another. This is easy to see using thinking, which takes a set of ideas and tests them against any given situation, but the feeling function seems less intuitive. In this case, the individual is still making judgments and value claims, but doing so through emotions. For example, an individual can correlate their feelings and a memory. If they see the same correlation existing in a future outcome, they can "rationally" use this comparison to reproduce a desirable emotional state.
Sensing and Intuition then are irrational functions as they are spontaneous and immediate. Sensing, for the most part, looks to the particulars of the outside world for information. By sensing what is going on in a situation, the individual can spontaneously see what is happening and then act on it. Intuition then relies upon personal, subjective information to make decisions. This very much correlates to Jung's idea of fantasy-thinking or imagination. Through this process, information spontaneously bubbles up from the unconscious, requiring no deliberation or processing.
I will leave these definition in this terse form because Jung does not classify each function alone, but in the context of introversion and extroversion. For example, Jung writes about introverted thinking and extroverted thinking as being fundamentally different types of thinking. Briefly put, the extroverted thinker tends to idealize objective reality, making themselves subject to objective laws that all must acknowledge. The introverted thinker, on the other hand, relies upon their understanding of the situation. Their theory is most definitely their theory although they claim it to have universal applicability in the same sense as the extrovert. The same follows to various degrees with feeling, sensing and intuition.
Jung breaks down his typology into multiplying his two attitude types by the four function types to get eight categories of personalities. This notably differs from the Myers-Briggs Type Indicator which includes a perceiving and judging orientation, bringing the personality types up to sixteen. Their addition to Jung's system was to assume that a person has both a rational and irrational function and whichever they favored dictated the emphasis on judging (rational) or perceiving (irrational).
In short, the General Description of the Types beautifully explains how Jung saw libidinal investment to be the cause a person's psychological orientation to the rest of the world. I think Jung did a brilliant job at personally explaining this original work in such a way that left little up to the imagination of the reader, which is a nice change of pace.