Found a wiki link:
Arrangement. I'm an fan of music, not a student, so a lot of this is over my head. It sounds like the arranger uses the original composition as a basis for development in a variety of directions. Is that a good general description? I'm trying to figure out who to credit for music that I really like. I tend to consider the composer the end-all-be-all but it sounds like the arranger can have a significant impact on the final piece.