"The Myers-Briggs Type Indicator (MBTI) assessment is a psychometric questionnaire designed to measure psychological preferences in how people perceive the world and make decisions." (Wikipedia)

I've used the instrument for years with a variety of students, and the insights and understanding it provides have always been welcomed by them.  While I have always treated personality testing as a sideshow, as a little something to garnish the material we cover in my courses and workshops, they nevertheless manage to take center-stage.  

You can do a free self-assessment at this website: Jung Typology Test (Human Metrics)

The Wikipedia link (along with its detailed sublinks) above provides a fairly thorough discussion and should be sufficient for most lay users of the instrument.

The Keirsey site is another excellent place for information on each personality type.  Keirsey, in fact, have developed and market the instrument (questionnaire) used to determine one's type.  A good place to start is the overview of the various temperaments.

Keirsey speaks in terms of four dimensions of personality based on type.  Read about them here.