There are two possible results for each one of the four letters in each type. The letters are:
E: Extraversion: Prefering the external world.
I: Introversion: Prefering the internal world.
S: Sensing: Preferingthe phsycial world.
N: iNtuition: Prefering the hypothetical world.
F: Feeling: Prefering emotionally driven choices.
T: Thinking: Prefering logically driven choices.
P: Perceiving: Prefering open ended results.
J: Judging: Prefering finality.
These 8 letters can be combined into 16 diferent ways in groups of 4 with one being from each pair to form the 16 types.