It's a weird theory, but it is somewhat coherent. INTJ is very close to INTP, just not really for that reason. ENTP is the classic entrepreneur. INTP/INTJ is the classic hacker; there was a poll on here and nearly 60% of the people 662 people who voted were INTP/INTJ, whereas they're each around 2-3% in the ordinary population. 10% of the rest were ENTP. (
http://news.ycombinator.com/item?id=204240)
Basically, the Myers-Briggs type determines your preferences for particular cognitive functions. The four letter indicator is like an extreme Huffman coding of those preferences:
Your information-gathering, perceiving function (that is, input.) This can be either i(N)tuitive or (S)ensing. Roughly, a sensing person trusts their senses, directly what they see and hear from the world. And an intuitive person trusts, or looks for knowledge, in their subconscious and conscious cognitive models.
Your decision-making, judging function (that is, output.) This can be either (T)hinking or (F)eeling. Roughly, the thinking person is at ease when they can make a decision supported by a logical process, and the feeling person prefers to make the decision according to their gut.
(E)xtroverted and (I)ntroverted describes the social attitudes of each of the 'functions,' in other words, whether you, when information-gathering, or decision making, prefer to focus on behavior, action, people and things or the internal world of ideas and reflection. There is only one letter for this, even though each function is supposed to have its own attitude. This is because Isabel Myers believed that there was always a balance; if you are introverted in information-gathering, you're extroverted in
decision making.
Finally, the last letter, (P)erciving or (J)udging, specifies which side of yourself; perception (information-gathering) or judgment (decision-making), you tend to show to the world. Perceiving types prefer to "keep decisions open." Judging types prefer to "have matters settled." The YCombinator practice of deciding quickly, while it may be a definite plus for both sides, is a judging characteristic, for example.