What it really comes down to is the external/internal focus of the different functions (a topic that I'm still trying to wrap my head around, so I apologize if I don't explain it right). The I--J tends to actually function more as a "Perceiver" than a "Judger", contrary to what you might think (and vice versa for the I--P), which is a common reason these two types are mistaken for each other. How it actually plays out though is quite distinctive, which is why once I started reading more about INFJ's, I realized how much more like me it sounded. Basically, there are...
"...two broad and fundamental options for approaching life and information: Judging and Perceiving. Perceiving involves a more passive and receptive approach to life. The first tendency of types with a dominant Perceiving function is not to control, but to allow. Judging, in contrast, involves a desire to control, order, and predict." (source here)
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Internal Intuition (Ni)
External Feeling (Fe)
Internal Thinking (Ti)
External Sensing (Se)
An INFP, on the other hand, has these functions:
Internal Feeling (Fi)
External Intuition (Ne)
Internal Sensing (Si)
External Thinking (Te)
And yes, the order of these functions is important (functions higher up on the list are more dominantly seen in a person's personality). More explanation of these stacking orders can be found here. To sum up this article, an Introvert's dominant (or top of the stack) function is always internally focused (whereas an Extrovert's dominant function is externally focused). Next, the types who last letter is "J" tend to extrovert their "Judging" function, which is actually the T/F part of the equation (confusing, no?). So in the case of an INFJ, my "Feeling" function is extroverted. For an ISTJ, for example, their "Thinking" function is extroverted.
For "P"s, they tend to extrovert their "Perceiving" function, which is the N/S part of the equation. So an INFP would extrovert their "iNtuition" function, and an ISFP, for example, would extrovert their "Sensing" function.
I haven't entirely figured out what goes into determining the order of these functions, but apparently the last function on the list is your least developed function and the functional opposite of your dominant function.
It's funny but a lot of the typical explanations for the INFP personality still sound a lot like me, but considering it from the point of view of whether my functions are external or internal makes it more clear that I am actually INFJ. Extroverted Intuition (Ne) for example (which is a trait of INFP) tends to cause someone to ramble aloud when they're processing something until they come to their conclusion. They may be seen as scatterbrained as they circle around clarifying in their mind what they're trying to say. An Introverted Intuition (Ni - a trait of the INFJ) tends to absorb the possibilities and then build on one singular idea inside their head before saying it out loud. They can often seem mystical or like a prophet when they say something because they can "jump" to a conclusion before other types, since they have done all their processing internally before speaking.
Internal Feeling (Fi - a trait of the INFP) tends to mean that a person's internal feelings are unswayed by external factors. So like, they may feel deeply about a certain topic or situation, but the fact that their feeling isn't shared by the others in the room is irrelevant. How they feel about a topic is deeply held and isn't likely to sway by external opinions and situations. For the trait of External Feeling (Fe - a trait of the INFJ), we tend to be like cold-blooded animals, in the sense that we absorb &/or mimic the feeling of a room. If I walk into a room of people that are angry, for example, I will instantly feel angry. If I walk into a room of people that are very excited, I'll tend to feel that way as well. It also works to where if I'm talking to someone, I can very easily put myself in their shoes and feel what they're feeling. I also tend to change my emotions based on what I see will help others in the room - I will exude calmness if someone needs calming down, for example, or exude anger if everyone around me seems overly lackadaisical about something that needs to be responded to more emotionally (this is largely an unconscious trait, not something I do purposefully). An Internal Feeler, on the other hand, is less concerned with cultivating the feeling in the room, but more concerned with helping people individually. They tend to be rescuers and also have a highly developed sense of their own worldview. They tend to take up causes that personally move them.
Internal Sensing (Si - a trait of the INFP) means that a person may fail to factor in external details as much. They tend to rely on past data to determine their current course of action, rather than taking into account current circumstances. They tend to avoid new sensations and experiences. An External Sensing person (Se - a trait of the INFJ) relishes new tastes, smells, etc, and are pretty in tuned to the world around them. They may notice a bird chirping outside the window before someone else might, for example. Their preferences for trends (in clothing, music, etc) also seem to be more dependent on what's trendy at the moment, whereas an Internal Sensor tends to stick with what's already tried and true (classic styles in clothing, for example).
External Thinking (Te - a trait of the INFP) tend to think out loud more than an Internal Thinking person. They may be more fact oriented, seeing things in black and white. They also tend to backtrack and try to clarify the question and underlying assumptions of something before actually trying to come up with an answer or solution. Someone with the Introverted Thinking function (Ti - a trait of the INFJ) tends to be pretty self-disciplined (able to resist eating an entire carton of ice cream in one sitting, for example) and tends to not express their opinions until they are a bit more fully formed. They are constantly pulling in new information to try to build up the theories in their minds on certain subjects. For example, I have running theories on personality types, birth order, learning styles, etc, and am constantly pulling in information to hone my viewpoint. An External Thinker would tend to already have a black and white idea of how something is, regardless of new information. While an External Thinker thinks more out loud, once they come to a conclusion, it tends to stay that way.
Anyway, sorry if this post is long (<-- an INFJ trait - I'm concerned about your feelings here, Lol). I guess it's an INFJ trait to want to teach people things (hence all my posts about blogging tips, small business tips, etc). Lol. I guess teaching you about personality traits is no different. I don't claim to be an expert here, but found all this research rather interesting and it's definitely changing how I'm viewing personality typing in general (<-- another INFJ trait - pulling in new information has morphed my opinions on something). Hopefully you've found this all rather interesting!