(taken from: http://www.personalitypage.com/html/INTJ.html)
As an INTJ, your primary mode of living is focused internally, where you take things in primarily via your intuition. Your secondary mode is external, where you deal with things rationally and logically.
INTJs live in the world of ideas and strategic planning. They value intelligence, knowledge, and competence, and typically have high standards in these regards, which they continuously strive to fulfill. To a somewhat lesser extent, they have similar expectations of others.
With Introverted Intuition dominating their personality, INTJs focus their energy on observing the world, and generating ideas and possibilities. Their mind constantly gathers information and makes associations about it. They are tremendously insightful and usually are very quick to understand new ideas. However, their primary interest is not understanding a concept, but rather applying that concept in a useful way. Unlike the INTP, they do not follow an idea as far as they possibly can, seeking only to understand it fully. INTJs are driven to come to conclusions about ideas. Their need for closure and organization usually requires that they take some action.
INTJ's tremendous value and need for systems and organization, combined with their natural insightfulness, makes them excellent scientists. An INTJ scientist gives a gift to society by putting their ideas into a useful form for others to follow. It is not easy for the INTJ to express their internal images, insights, and abstractions. The internal form of the INTJ's thoughts and concepts is highly individualized, and is not readily translatable into a form that others will understand. However, the INTJ is driven to translate their ideas into a plan or system that is usually readily explainable, rather than to do a direct translation of their thoughts. They usually don't see the value of a direct transaction, and will also have difficulty expressing their ideas, which are non-linear. However, their extreme respect of knowledge and intelligence will motivate them to explain themselves to another person who they feel is deserving of the effort.
INTJs are natural leaders, although they usually choose to remain in the background until they see a real need to take over the lead. When they are in leadership roles, they are quite effective, because they are able to objectively see the reality of a situation, and are adaptable enough to change things which aren't working well. They are the supreme strategists - always scanning available ideas and concepts and weighing them against their current strategy, to plan for every conceivable contingency.
INTJs spend a lot of time inside their own minds, and may have little interest in the other people's thoughts or feelings. Unless their Feeling side is developed, they may have problems giving other people the level of intimacy that is needed. Unless their Sensing side is developed, they may have a tendency to ignore details which are necessary for implementing their ideas.
The INTJ's interest in dealing with the world is to make decisions, express judgments, and put everything that they encounter into an understandable and rational system. Consequently, they are quick to express judgments. Often they have very evolved intuitions, and are convinced that they are right about things. Unless they complement their intuitive understanding with a well-developed ability to express their insights, they may find themselves frequently misunderstood. In these cases, INTJs tend to blame misunderstandings on the limitations of the other party, rather than on their own difficulty in expressing themselves. This tendency may cause the INTJ to dismiss others input too quickly, and to become generally arrogant and elitist.
INTJs are ambitious, self-confident, deliberate, long-range thinkers. Many INTJs end up in engineering or scientific pursuits, although some find enough challenge within the business world in areas which involve organizing and strategic planning. (NOTE: This is just what I'm interested, not what I do.) They dislike messiness and inefficiency, and anything that is muddled or unclear. They value clarity and efficiency, and will put enormous amounts of energy and time into consolidating their insights into structured patterns.
Other people may have a difficult time understanding an INTJ. They may see them as aloof and reserved. Indeed, the INTJ is not overly demonstrative of their affections, and is likely to not give as much praise or positive support as others may need or desire. That doesn't mean that he or she doesn't truly have affection or regard for others, they simply do not typically feel the need to express it. Others may falsely perceive the INTJ as being rigid and set in their ways. Nothing could be further from the truth, because the INTJ is committed to always finding the objective best strategy to implement their ideas. The INTJ is usually quite open to hearing an alternative way of doing something.
When under a great deal of stress, the INTJ may become obsessed with mindless repetitive, Sensate activities, such as over-drinking. They may also tend to become absorbed with minutia and details that they would not normally consider important to their overall goal.
INTJs need to remember to express themselves sufficiently, so as to avoid difficulties with people misunderstandings. In the absence of properly developing their communication abilities, they may become abrupt and short with people, and isolationists.
INTJs have a tremendous amount of ability to accomplish great things. They have insight into the Big Picture, and are driven to synthesize their concepts into solid plans of action. Their reasoning skills gives them the means to accomplish that. INTJs are most always highly competent people, and will not have a problem meeting their career or education goals. They have the capability to make great strides in these arenas. On a personal level, the INTJ who practices tolerances and puts effort into effectively communicating their insights to others has everything in his or her power to lead a rich and rewarding life.
If you'd like me to do this same thing for other types, just let me know.