Temperament is a behavioral model developed by David Keirsey to classify people into one of four basic patterns of behavior.
Although a separate theory, the Temperament model draws upon the MBTI instrument to identify the four groups: Idealists (NF), Rationals (NT), Guardians (SJ) and Artisans (SP).
Temperament by-passes some of the complexities of Type and becomes an actionable tool by focusing on observable, and even predictable, behavior.
All 16 MBTI "Types" fit into one of the four Temperament categories using the combinations illustrated below. If a someone has a preference for Sensing as their Percieving Function, then their Temperament will be identified by their Outer-World Orientation (either Judging or Perceiving); if a someone has a preference for iNtuition as their Percieving Function, then their Temperament will be identified by their Judging Function (either Thinking or Feeling).
The best resource for understanding Temperament and its relationship to Type is Keirsey's own website, but have some of my own information linked above as well.