Each of the four-letter combinations in this wiki refers to a distinct personality type, based on the work by Katharine Cook Briggs and Isabel Briggs Myers. The first letter can either be an I, standing for "introvert," or E, for "extrovert." The second letter is either an S, for "Sensing," or N, for "Intuition." The third letter either a T, for "Thinking," or an F, for "Feeling." The fourth and final letter is either a J, for "judging" or P, for "Perceiving.
The I/E Dichotomy
In this theory, Introversion and Extroversion has nothing to do with how outgoing, shy, withdrawn, gregarious, or personable someone is. Instead, an Introvert directs their attention and derives their energy from within, while an extrovert directs their attention and derives their energy from the world around them. An introvert may prefer to reflect, philosophize, create art, read, or do other things alone simply because it's easier and more enjoyable for them to do those things than interact with many people. On the other hand, an extrover may like to go out to parties, meet many new people, experience new things, or otherwise surround themselves by lots of stimulation. It is important to keep in mind, however, that while activity preferences are secondary and may often suggest that one is an introvert or extrovert, in reality only the individual knows where it's easier direct and derive their energy.
The S/N Dichotomy
Sensing and Intuition refer specifically to how one gathers information about the outside world. S-types will gather information using their five senses. This may mean that they observe the world around them in very keen detail, but it may also mean that their sensual information when recalling past experiences is crisp and easy to recall. It also means that Sensors deal better with facts and concrete objects than they do with abstractions and "general directions." The things that a Sensor knows, they will know clearly, and be able to describe in literal, easy-to-understand language.
Intution instead gathers trends of information, observing the relationship between ideas and objects, rather than the ideas and objects themselves. It much more easily derives meaning from all things, and lives much more of its life in the future or the possibilities of how things could be. Intuitive types may gather and understand immense amounts of information about ideas and how they relate, but neglect what's right in front of their faces. Opposite to Sensors, Intuitives deal better with abstractions, ideas, and general pictures than they do with facts and concrete things.
Still not sure if you're an S type or an N one?
Visit the Cognitive Functions page and read about the Perceiving functions (Si, Se, Ni, Ne), and decide which one of those four best describes how you gather information. If the answer is Si or Se, you're a Sensor. If the answer is Ni or Ne, you're an Intuitive.
The T/F Dichotomy
Thinking and Feeling refer to one's method of making decisions. The concepts these words refer to are not the same as their everyday usage, however. Feeling does not always pertain to emotions, and feelers may spend hours thinking about a certain problem. Likewise, thinkers may make hasty decisions quicker than is prudent.
Instead, Thinkers will want to dig into an idea to find a universal principle that applies and use it asmany decisions. Their decision-making is impersonal and objective, since they try both to let their own and other peoples' wishes have no bearing on their decisions. They say what is truthful, exactly how they see it (unless lying, of course), instead of first sugar-coating a response and wondering how it affects the person they're talking to, and expect other people to do the same for them.
Feelers, on the other hand, make their decisions based carefully considering how what they say will affect the other people involved--often even if it means not pointing out a big mistake that someone's making. They have an almost-toxic response to conflict in their relationships, and generally care more that the relationships aren't harmed than anything else, unless their other ideals interfere in the relationship-damage calculation somehow.
Still not sure if you're a Thinker or a Feeler? Visit the Cognitive Functions page and read about the Judging functions (Ti, Te, Fi, Fe). If the one that best describes how you make decisions is Ti or Te, you're a Thinker. If the one that best describes how you make decisions is Fi or Fe, you're a Feeler.
The J/P Dichotomy
Judging and Perceiving are two different ways in which people like to interact and orient themselves with respect the world. Like Thinking and Feeling, however, these words do not refer to the same concepts that most people associate them with when they use these words.
Instead, Judgers interact with the world by organizing it in some way. In a word, they are decisive. J-types become uneasy when things are undecided, plans aren't finalized, lists aren't made, or no definite goal is in sight. Keep in mind: this only refers to how they orient themselves with the outside world and external things. Internal things may be still be disorganized, undecided, and flexible without any trouble at all, because a Judger will draw their information from within and use it to structure the world outside.
Perceivers, on the other hand, are the exact opposite. They draw on their information from the outside world, and use that information to structure their internal ideas or values or selves. Instead of being decisive, Perceivers are speculative. The condition that Judgers crave--the fixed goals, lists, and detailed plans for action--may actually constrict a Perceiver and inhibit their ability to gather information. For this reason, Perceivers remain spontaneous, may change their plans (on what to do, in the outside world) quite easily, maintain flexibility, and adapt to external changes easily. As with Judging, these qualities are only meant to describe how a Perceiver interacts with the external world and ideas. Internally, they may still be very structured and definite in their thoughts.
To put shapes to the concepts: Judgers thinking a very linear fashion. Once a goal is in mind, a J-type figure out the steps that have to be taken, and gets to work, one step at a time. a P-type, on the other hand, things quite circularly. Once a goal is in mind, they may look at the world around them and figure out a few paths that may yield progress towards a solution, and do a little bit of work on each one. They then reevaluate where they are in relation to their goal, and do a bit more work on new paths that look like they may yield progress. In this way a Perceiver's work is much less efficient, but they gain the advantage of learning many unexpected things along their way to a solution.
A Common Misconception
It is important to realize that while most people consider these four-letters the types in themselves ("I 'am' an introvert"), the four letters are actually a shorthand for underlying cognitive functions, which were first theorized by Carl Jung.
Jung suggested that everybody uses the same basic four cognitive functions, except with different strengths and orientations. These functions are called Thinking, Feeling, Sensing, and Intuition. These functions can be ordered by strength of preference, and oriented in various ways (introverted/extroverted) to create the various different personality types.
Notation and Names
To help save time, each function/orientation combination is usually referred to by a two-letter abbreviation. The first letter is the capital letter which identifies the function. Sensing = S, Thinking = T, Feeling = F, Intuition = N. The second letter is a lower-case letter which describes the orientation of that function. Introverted Thinking, for instance, would be referred to as Ti. Extroverted Intuition, on the other hand, would be called Ne. The other functions are named similarly.
Whichever function a type prefers to use the most is called "dominant", while the second strongest is called "auxiliary." Together, the dominant and auxiliary functions make up the "primary axis." The third-strongest function is called "tertiary," while the weakest one is called "inferior."
Thinking and Feeling are called Judging functions. These relate to our decision making process. Sensing and Intuition are called Peceiving functions. These relate to our information gathering process.
A function is another function's Opposite if the two are of the same type (both Judging, or both Perceiving), and different orientation. So Te is the opposite of Fi, and Si is the opposite of Ne.
The theory says that everybody of every type uses all of these functions. Both our senses and intuition help gather information, and we use both our thinking and feeling to make decisions. The difference between two types is which way these functions are oriented (introverted or extroverted), and which order we prefer to use them. Nobody can use both functions of the same type as the same time. That is, if someone is making their decisions using their Thinking function, then they can't make the same decision using their Feeling function. If someone is gathering information by Sensing, then they cannot also gather information with Intuition at the same time.
The primary axis is the function set through which a type prefers to act. Every type's primary axis will contain one Judging function and one Perceiving function. The rationale behind this is because a type with two Judging functions on top would be unable to gather information sufficiently, and a type with two Perceiving functions on top would be unable to make decision effectively... so the theory says that it won't happen in any normal case. For a similar reason, one function in the primary axis will be extroverted, while the other function will be introverted.
The final piece of the theory has to do with the ordering of the functions. Since personality is all about preference, and you cannot use two functions of the same type at the same time, we are constantly making choices as to how we want to think. The more heavily we favor Thinking, the less we will favor Feeling. The more heavily we favor Intuition, the less likely we are to use Sensing. What this means is that, for all types, the inferior function is the Opposite of the dominant, and the tertiary function is the Opposite of the auxiliary. This also ensures that people are "whole"--with both one introverted and one extroverted Judging function, and one introverted and one extroverted Perceiving one.
Converting from the Four Letter Code to Cognitive Functions
1. The middle two letters (SF, ST, NT, or NF) describe the functions of the primary axis. The fact that the dichotomies are S/N and T/F reflect the theory so that each primary axis has one Perceiving function and one Judging one. It does not tell you which one is stronger, or anything about how they are oriented.
2. The last letter (J or P) describes which of the functions in the primary axis is extroverted. A P type will have an extroverted Perceiving function, while a J type will have an extroverted Judging one. As a result, the "other" function in the primary axis will be introverted. An xSFJ type, for instance, will have an Fe and Si function, while an xNFP will have an Ne and Fi function. This still tells you nothing about the order of strength.
3. The first letter (I/E) tells you whether the dominant function is Introvereted or Extroverted. Using the example from #2, we can see that the primary axis of an ENFP will be Ne-dominant, Fi-auxiliary, and the primary axis of an INFP will be Fi-dominant, Ne-auxiliary.
4. The inferior function is the Opposite of the dominant function, while the tertiary is the Opposite of the auxiliary. So an ENFP, who is Ne-dominant, will be Si-inferior, and having an Fi-auxiliary means that they will have Te as a tertiary.
Using these same four principles, it is easy to determine the cognitive functions of any given type.
Thinking in terms of the cognitive functions is also more helpful, because the four-letter system suggests that "all differences are equal." That is, ENFP appears to be one letter different from both INFP and ENFJ. If you look at their functions, however, you can tell that there is a noticeable difference beteween the two. Compare...
INFP - Fi, Ne, Si, Te
ENFP - Ne, Fi, Te, Si
ENFJ - Fe, Ni, Se, Ti
What appears to just be one letter difference can actually make quite a difference. The E/I switch merely shuffles around the strength of preference a little bit, but the J/P switch changes both the power and orientation of all the functions.
Finally, thinking about Type in terms of the cognitive functions is useful because it gives us a more precise theroetical standpoint if we are to examine a personality. How does an ENFP work? What makes Extroversion combine with Perceiving, and what characteristics does it produce? These questions are better answered by examining the cognitive functions, which reveals that Extroversion is similar between ENFPs and ENTPs (both Ne-dominant), but much different than the extroversion of ESTJs (with a dominant Te). Instead, we can learn about Ne itself, by studying the common-ground of ENFPs and ENTPs, and then extrapolate when we ask "now how would this react if subordinated to Si?" -- to understand the effects of the less-visible, underlying functions of an xSFJ type.
For more information concerning the individual cognitive functions, click here.