DivX is an extremely popular implementation of the MPEG-4 standard -- probably 50% or more of the movies available in the Net have been encoded with DivX today. It means basically that you need to have this software installed in your computer if you want to watch or encode DivX movies. This is the compiled Apple Macintosh codec version of the implementation and it has been developed by the same company who has developed also the Windows version of DivX5.