INF is the "intuitive-introvert-feeling" type as indicated by the Myers-Briggs Type Indicator.

Some info: http://www.myersbriggs.org/my-mbti-personality-type/mbti-basics/index.asp

Wikipedia entry: http://en.wikipedia.org/wiki/Myers-Briggs_Type_Indicator

More stuff: http://www.teamtechnology.co.uk/tt/t-articl/mb-simpl.htm

Here's an online test: http://www.humanmetrics.com/cgi-win/JTypes2.asp