between and among, I believe. In other words, something is located BETWEEN (INTRE) two things, but AMONG (PRINTRE) three or more things. I'm pretty sure that's right.
I recently saw an explanation posted by another user on a sentence in this lesson, which summed it up perfectly. Here's a brief transcript of what he said: între, printre = finding something amongst other things that are not similar (a horse amongst apples) dintre = finding something that are similar (the most beautiful girl between all girls) If you find him, give him a Lingot :)