Philosophers through the ages have repeatedly grouped innate dispositions into just a handful of categories. From as far back as two thousand years ago, excesses of blood, black bile, yellow bile, or phlegm were thought to lead to cheerfulness, somberness, enthusiasm, or calmness. While more contemporary psychologists like David Keirsey moved past the emphasis on body fluids, they seemed to all speak categorically about different but still respectable styles of being human.
Keirsey lends significance to this as he writes in Please Understand Me: Character and Temperament Types, "Suppose it is so that people differ in temperament and that therefore their behavior is just as inborn as their body build. Then we do violence to others when we assume such differences to be flaws and afflictions."
Keirsey was inspired by the work of Isabel Myers and Katheryn Briggs whose personality-typing instrument measured an individual's preferences for Carl Jung's four cognitive functions: the perceiving functions of Sensing (S) and Intuition (N), and the decision-making or judging functions of Feeling (F) and Thinking (T). The Myers-Briggs Type Indicator provided Keirsey with a field of attributes with which to differentiate four broader categories of motivating temperaments: the SP Artisan, SJ Guardian, NF Idealist, and NT Rationalist. As we all have access to each of Jung's four functions, the word preference is key.
Regardless of whether we're born with preferences or acquire them sometime in our youth, the fact is, we wind up more adept with some functions than others. While we may not be the best at handling details, for instance, we may be great at seeing the bigger picture. We can allow these differences to feed frustration and miscommunication with people having other preferences, or we can choose to instead focus on how we might compliment one another.
As Isabel Briggs Myers and Peter Myers write in Gifts Differing: Understanding Personality Type, "When two people reach a deadlock over the handling of a given situation, the trouble may be a result of their difference in type, which has interfered with their communication. When two people have not known the same facts, or not considered the same possibilities, or not foreseen the same consequences, each has only an incomplete knowledge of the problem. They must put it all together. Each needs to use all four processes, however well or ill developed: sensing to gather the relevant facts, intuition to see all measures that might usefully be taken, thinking to determine the consequences, and feeling to consider the impact of these consequences on the people involved. The pooling of their respective perceptions and judgments offers the best chance of finding a solution valid for them both."
Additional information can be found on the Wikipedia page on Temperament and Type.