http://www.humanmetrics.com/cgi-win/jtypes2.asp
Quite a renowned and passed-about thing online, I'm sure there've been threads about it on here before.
Just adding a poll to see how TiBB's made up
Key:
E = Extroverted
I = Introverted
S = Sensing
F =Feeling
J = Judging
T = Thinking
P = Perceiving
N = Intuitive
And a fun graph someone made using Harry Potter characters to explain it a bit (in the easiest terms possible...)