I just now noticed that there is alot of disagreement and speculations about what the definitions of the functions mean. I'm not going to say that this is going to clear up any confusion but it might actually help a little bit in understanding what the functions are fundamentally. The above link is from the International Socionics Institute page and describles the "Qualia" of the symbols used for each of the 8 functions.
I don't think its as philosophical as it looks at first.
I shall summarize:
Perception= Time and Space
Judgement= Matter and Energy
Time, space, matter and energy. These are the four elements which describe the 8 functions. When a function is introverted it appears to form correlations and associations with past experiences and memories, when a function is extraverted it correlates with actual experience, what is going on in the here and now.
So one can deduce that introversion corresponds with associations, extraversion corresponds with practicality, what is real and tangible.