What do the letters stand for?
It's MBTI typology. I actually think it's blown out of proportion on the internet, with websites left and right providing inaccurate tests that don't even seem to grasp the complex structures that underlie it all. Most people see it as a generalizing form of entertainment now, much to my regret. The websites that actually go in-depth about the individual functions are few and far between, with some sources even being biased. I've done a lot of research about it, because I see a lot of potential in it.
The basic idea that is actually quite complex to explain, is that there are 8 psychological functions, which everyone carries. Every person has a certain preference regarding these functions. The more you use a certain function, the better your skills become of applying that function in real life. That is how you learn, and how a child grows into an adult psychologically. Myers-Briggs built upon Jung's theories and developed a structure of 16 different profiles, each with 4 of these functions, in which they continuously alternate each other. They do this to form a balance. Usually, the preferences and skills regarding the functions of an individual are not 100% fitting to these profiles. Because everyone contains all 8 functions, it may be seen as bit superficial to only note 4 of them. But it's true that we can only learn to a certain extent in our lives, and learning to use all 8 functions correctly is basically impossible. The profile that is assigned to an individual (if you have a good test, that is), thus focuses more about obtaining balance, and building that balance upon the basis of your best developed two functions. The other two functions in the profile should help you surmount obstacles, and lead you towards a more balanced life.
The functions are:
Se: extroverted Sensing
Si: introverted Sensing
Ne: extroverted iNtuition
Ni: introverted iNtuition
Fe: extroverted Feeling
Fi: introverted Feeling
Te: extroverted Thinking
Ti: introverted Thinking
The sensing and intuition function are observing in nature, while the feeling and thinking function are judging in nature. You need both, because if you only have judging functions you will only ever be biased and won't be able to regard matters realistically or open-mindedly. If you would only have observing functions though, you would never be able to make decisions, because these come from reason and feeling, from wanting something through bias.
Every function has an extroverted and an introverted variant. This is mainly in regards to direction. While Te may think a lot about the external world and broad structures, Ti is more focused on thinking and analyzing everything about themselves. Fe is more focused on caring for others, while the most important aspect of life for a person with high Fi is following their own values and realize their own happiness. Si types will be the ones that would perceive hunger inside themselves, while Se types are more likely to have cravings for pizza. Si types have incredible memory, because this function is the one that bodily sensors use when backtracking within themselves, trying to revisit spots inside the brain they have visited before, but without the external sensor available anymore. Se types live in the present and are very down to earth, because they mostly sense everything around them. Ne types are open minded and have many ideas, which usually come to them through interaction and free associations with the outside world. Ni is open minded as well, but the ideas are fewer and a lot less flighty, more intense. They come to them, usually very sudden, at moments where they are alone or feeling at ease.
As far as the balance in the profiles go, I can give you an example. The profile assigned to me is INTP. The four functions of INTP in order of how developed they are in me are: Ti, Ne, Si, Fe. You might see a pattern here. Two objective functions, one to perceive the inside world and one to perceive the outside. Two judging functions as well, one to judge myself, the other one to judge others. One for ideas and abstract thinking, one for realism and taking care of your physical health, one for rational thinking, and one to feel emotions. That's the balance, that's why there are only four functions required in a profile. To take this example further I can tell you that I am very adapt at Ti and Ne, and happen to have a strong Te and Fi as well. But as you may notice, these are almost exclusively judging functions, and there are absolutely no sensing functions so far that I really feel I'm skilled at. This means that I have trouble being realistic, and my ideals can drive me to harsh judgements that make my life very painful indeed. I always feel like I'm failing. Luckily, my INTP profile shows me the way out. I have spent too many years on a square meter analyzing myself, and I've noticed that I feel much better if I actually
do things in the outside world. Si and Se are both about doing things, but another major aspect I need to learn from Si is to respect the past. I throw away things very easily, but if I would hold onto them for a little longer, I might learn some more. Knowing this, I can find motivation to go outside, take better care of my body, give things a second chance, accept my past experiences, and so on. That way I feel healthier and more alive in general. The lowest developed function in my profile in Fe. I have Asperger's and am not good at social situations at all. But I can learn. Even a diagnosis like Asperger's isn't going to hold me back, because even though I know I will never be a 'social butterfly' or anything like that, I do care about people. The more I communicate feelings and the more I learn from past mistakes, the better I will grow.
If you want to read more about MBTI, I recommend the following websites:
Keys2Cognition - This website may not look like it, but it has the best and most extensive MBTI profile test on the web, which takes into regard all 8 functions.
BestFitType - This website explains the functions very accurately.
A word of advice:
Stay away from any websites with MBTI profile descriptions, they tend to be very generalizing. The 8 functions can be applied very broadly and abstract, so it really depends on your own situation and life experiences how these functions manifest themselves in you. With such limited good sources available, it is best to stick to the fundamentals, the two websites I have linked above, the 8 functions, and your own abilities to draw conclusions from these.