What are codec and xvid? I could not play the movies I've downloaded through torrent

using DivX, and Windows? I've been using torrent lately to download movies. I was able to download a few movies last 3 days ago but unfortunately I couldnt play the files which usually are in xVid avi format. I'm using DivX, BSplayer and Windows Media player to view the files but all of them are telling me that the files I downloaded have codec error. Please help.