I was curious about this subject as well. Here is how I understand it... (not sure if its right, but I like this explanation)...
A Chorus is the repeating hook of the song that has its own unique melody and structure.
A Refrain is the repeating part of the song, but sung, and played, very similar to the verse.
So what I mean by this is that the refrain could just be another verse, except you repeat it more than once (usually between each verse). While a chorus is not just different words set to the same, or similar tune, it's unique and not the same melody as the verses.