MBTI is one of the many methods to judge your personality type. It uses 4 letters with 2 possibilities each to define the 16 types. When put together, the 4 letters make up a unique (you and a few thousand other people) personality.
Quoted right off the official website about what each letter means:
Information pages can tell you about some quirks of each type, as well as the possible careers for the person. The 16 Personalities website even has things from work habits to romantic relationships. Of course, this is not entirely accurate. It's a standardized test; it doesn't define you, just the result you get. You may be a borderline person, able to fit in either of the two possibilities. You may take this test a second time, only to find out you got a completely different result. The purpose of this indicator is simply to... Generalize. It makes finding potential interests easier and it helps puts a label on things.
Take the test: http://www.16personalities.com/free-personality-test
For more information: http://www.myersbriggs.org/ or http://www.16personalities.com/
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Last year, our class all took this test for a project on our future career. I got ENTP, the "devil's advocate" who can't stick to schedules. This year (as in today), I took it again and I got... ISTP. Huh.
EDIT: You can have multiple choices, but please keep it between 2-3 at the most.