A system of personality classification developed by Kathleen Briggs and Isabel Myers way back in the 20th century. Claims to be based on Carl Jung's ideas in his book Psychological Types, but radically revised by David Keirsey to be more in line with a great many other theories about personality going all the way back to ancient Greece.
The Myers-Briggs Type Indicator (MBTI) is a test, devised by Briggs and Myers, which you have to pay to take. Don't call it a "test" around people who give the test, though. They'll insist that it's an "indicator".
The Keirsey Temperament Sorter (KTS) is a different test, based on Keirsey's redefinitions of all the terms, which is available on-line for free (see below).
There is a second level MyersBriggsTwo?
http://sunsite.unc.edu/personality/faq-mbti.html
Consult the MyersBriggsTypes, for reference.
See MyersBriggsDiscussion for a discussion of the validity and/or usefulness of the MyersBriggs personality test.
See MyMyersBriggsTypeIs... for quick statements of your test results.
What it is:
I would suggest consulting Steve Myers (no relation) and his viewpoint on contribution by type to the team:
and -- Dave Fischer
See also LipsonShiu
Book: Managing psychological factors in information systems work
Moved very insightful and elaborate summary to MyersBriggsForProgrammers.