A file with the extension .divx has been encoded with the DivX codec. DivX is an MPEG-4 format, therefore usually producing high quality video files at lower birates, especially from a DVD source. Most files encoded with DivX nowadays have a .avi extension but there are still plenty of .divx files floating around P2P networks.
You need to download and install the DivX codec to play back these files.
