  A while back I wrote urlLink a post introducing what personality types are. Here I will continue the introduction, by explaining the different functions. Be sure to read that earlier post first, if you have no background on what the four preferences are. Each personality type breaks down into four distinct functions, based on the four preferences of that type. What is a function? A function is how a person interacts with the world. It is very important not to confuse preferences and functions. Preferences are the four letters that make up a type's name. Functions are how those preferences combine to explain how a type, well, functions. Each function is made up of one of choices for the middle two preferences in a personality type's call letters. That means Sensing, iNtuition, Thinking, and Feeling. These are represented as capital letters in the shorthand notation.
Each of these also has a lower case letter, signifying it's direction: whether that function is introverted or extroverted. The four functions for INTPs are: Ti Ne Si Fe. In that order, from dominant, to inferior. Translating, they are: introverted Thinking extroverted iNtuition introverted Sensing extroverted Feeling Using my type as an example, you can notice a few things. First, is that the dominant (top) function, is the same direction as my 1st preference: Introverted (I). You will also notice that the top two functions are determined by what the middle two preferences are: iNtuition, and Thinking (N and T). Finally, and less obvious, is the fact that as the 4th preference is Perceiving (P), that means that the 2nd Preference, iNtuition in my case, is extroverted (Ne). If my 4th preference was Judging, then the 3rd Preference would be extroverted. To see what happens when just the 4th preference is switched, lets look at INTJs. Ni Te Fi Se introverted iNtuition extroverted Thinking introverted Feeling extroverted Sensing Again, the 1st preference is the same as the 1st function: Introversion. This time though, as a Judging personality, the Thinking is extroverted, and as such is the 2nd function in INTJs, where the iNtuition is the primary function.
You will note that as the top two functions switched places and directions, the bottom two did as well. This is due to the fact that the first and last function, are always from the same preference. Either the perceiving preference, or the judging. The middle two functions also follow this rule. Also, you will note that the direction of the functions always alternates from one to the other as you go down the function list.
Introverted, then extroverted, then introverted, then extroverted again. It is the opposite, but still alternating, order for those with the Extroverted preference. Here are a few examples to see if you correctly understand how the function order is determined. ESFJ: Fe Si Ne Ti ENTJ: Te Ni Se Fi ISTP: Ti Se Ni Fe ESFP: Se Fi Te Ni Later, I will explain what each of those functions mean, and how they work in each type. 
