A personality test, more or less. Developed by David Keirsey in 1978 and described in the book Please Understand Me and its later follow up, Please Understand Me II. An online version of the Keirsey Temperament Sorter can be found at http://www.advisorteam.com/user/kts.asp

There are four different scales that the personality is classified by:

Extraversion/Introversion (E/I)
Sensation/Intuition (S/N)
Thinking/Feeling (T/F)
Judgement/Perception (J/P)

The different base classifications are as follows:

Artisan(SP)
Guardians (SJ)
Rational (NT)
Idealist (NF)