It's based on the theory of personality types worked out by C. G. Jung. I was really into Jung years ago. It's probably not an exageration to say that his writings changed my life, so I was really excited to find this test. Although Jung developed the theory, he never formulated an actual test. He simply got to know his patients through analysis and learned to spot the different types.
Here's how it works. There are four basic psychological modes humans use to relate to others and their environment: thinking, feeling, sensation, and intuition. Everyone draws on all four to some degree, but different individuals have them developed to different degrees. Everyone has a primary one, which is the most developed, and a secondary one, which supplements the first but is not as developed. The other two are often barely developed or drawn upon at all.
People whose primary mode is thinking are extremely rational. They use logic and do things that make sense and can be explained. They think things through and are very good at spotting inconsistencies.
Feeling people tend to go with their gut. They avoid doing things that just don't feel right, and they often judge people based on a feeling. Things are seen as fitting somewhere on a scale between very bad and very good.
Sensation emphasizes perception. People in this category are the ones who like to say that "seeing is believing". Most of the information they draw upon comes from firsthand observations. These are not people who rely much on books to teach them about the world
Those who are intuitive are fairly confident that they know how the world works. They have a framework built up in their minds, and anything they experience is seen as fitting into that framework. Their actions and important decisions are dictated far more by what fits in their worldview than by logic, and a great deal of it is subconscious.
You may want to look up other descriptions of these four functions. I find that different people have radically differing ways of describing them. Generally, a person will speak highly of the functions that are dominant for them and disparagingly of the ones which are least developed in them, usually without even being aware that they are doing this. I met one woman who, in a presentation, was going over the strengths of each one, and she had a heck of a time thinking of even a single good thing to say about people who are thinkers. That really blew my mind, especially since the point is that none is superior to any of the others. Ideally we should try to develop each one as much as possible, so they can compliment one another.
In addition to the four functions, most people can be classified as either mainly introverted or mainly extroverted. (Jung is the one who invented those terms.) Extroverts are mostly preoccupied with things and people in the external world. Introverts are more internal. Once again, they are not mutually exclusive, but most people tend to be much more one than the other. (Introverts are probably much more interested iin taking this test than extroverts.)
This test also adds a distinction between judging and perceiving. These aren't Jungian distinctions, and I'm not certain I understand them, so I won't attempt to explain them.
My test results are posted at the top of this entry. Tomorrow I will write about what they say about me. In the meantime, I invite you to go take the test and find out just what kind of person you are.