It goes back to psychoanalysis a hundred years ago. Carl Jung, one of the major figures in that field, found out that we basically have four cognitive functions on which we operate and that we process information in a certain sequence. There is Feeling, which refers to “global thinking,” meaning you include all your life experiences and your values (your ethical system), intuition (situational thinking, including your subconscious), Thinking (step-by-step, conscious, logical analysis), and Sensing (immediate sensations). They can be of the same intensity for someone, but they are “called up” in a certain order, i.e. e.g. “Feeling” personality types evaluate a situation usually first according to their ethical point of view.
Imagine your mind being like a small company of four people representing the four functions. The job goes to #1 always. He might pass it down to #2, etc. Employee #4 might be great, but most of the times he won’t receive any work or has to wait until the work is passed down to him. Different people have a different sequence of functions, and each function can be directed to the inside and to the outside. In the end, you have about 16 different types, with some much more prevalent than others (more about MBTI here).’