C.G. Jung's theory of psychological types suggests that human behaviour is not random, but is the result of a few basic, observable differences in mental functioning. These differences are centred on how people prefer to use their minds, specifically how they choose to perceive and make judgements. He identified four functions at work: perception via sensation or intuition, judgement via thinking or feeling.

The MBTI includes four types of distinct preferences:

Index Preferences between	Affects choices as to
EI (E) Extraversion or     (I) Introversion	Whether to direct perception judgement mainly on the outer world (E) or mainly on the world of ideas (I)
SN (S) Sensing perception     (N) Intuitive perception	Which kind of perception is preferred when one needs or wishes to perceive
TF (T) Thinking judgement     (F) Feeling judgement	Which kind of judgement to trust when one needs or wishes to make a decision
JP (J) Judgement     (P) Perception	Whether to deal with the outer world in the judging (J) attitude using (T or F) or in the perceptive attitude (P) (using S or N)

Know Your Type at KnowYourType.com personality testing
Understanding your MBTI at PersonalityPathways.com
Suggested Careers for Myers-Briggs Types

Results Analysis:
Summary Here
Characteristics Frequently Associated with each MBTI Type at AnalyticTech.com
The 16 MBTI® Types at MyersBriggs.org
Robert Munafo's MBTI Proximity Chart - A Karnaugh map of the Myers-Briggs personality types
MyPersonality.info
keirsey.com