Swiss Psychiatrist Carl Gustav Jung (1875 - 1961) developed a personality theory at the beginning of the 20th century. He observed and explained patterns in seemingly random individual behavior.
Jung's first observations revolved around two ways people engage with the world.
He defined the term Extraversion (in the MBTI results described with an 'E') for people who gain energy by relating to the outside world.
He defined Introversion (in the MBTI results described with a 'T') for people who gain energy by focusing on their own internal world.
Extraversion does not mean exaggerated, Introversion does not mean shy. The terms describe where our mental energy flows, and are also referred to as an "attitude".
Jung continued, stating that our brain activity is mainly engaged in one of two things: taking in information (a process he called Perception), or making decisions based on the information we have taken in (which he called a Judging process). These two processes are also referred to as the cognitive or mental functions.
Jung describes two forms of taking in information: Sensation (aka Sensing) 'S' or Intuition 'N'.
People who prefer Sensing 'S' tend to trust information from their five senses. They prefer detailed information about the here and now, as well as practical application. Introverted Sensing 'Si' is focused on past experiences and reviewing, Extraverted Sensing 'Se' is focused on experiencing the surroundings in the moment.
People who prefer Intuiting 'N' tend to find patterns and themes in the information they gather. They prefer general overviews and find possibilities of what the information might mean for future development. Introverted Intuiting 'Ni' is focused on a vision of what might be and foreseeing, Extraverted Intuiting 'Ne' is focused on future possibilities and brainstorming.
Sensing does not mean sensitive, Intuiting does not mean intuitive. The terms describe how we use our brains to take in information.
Jung described two forms of decision-making: Thinking 'T' or Feeling 'F'.
People who prefer Thinking 'T' tend to make rational decisions based on logical objective analysis, considering the system and connected frameworks, and may not shy away from a debate. Introverted Thinking 'Ti' focuses on defining principles and analyzing, Extraverted Thinking 'Te' focuses on organizing and systematizing.
People who prefer Feeling 'F' tend to make rational decisions according to the framework of their values, how the decision might impact the people involved, and may prefer to have consensus and maintain harmony. Introverted Feeling 'Fi' focuses on clarifying what's important and valuing, Extraverted Feeling 'Fe' focuses on harmony and connecting.
Thinking does not mean rational, Feeling does not mean emotional. The terms describe how we use our brains to make decisions.