.Xvid-RIVER.avi file wont play in QT7

I have a movie that has the file extension .Xvid-RIVER.avi and it wont play in QT7.

The warning message that appears when I try to play it says that I need a codec and to look on the QT web page for it. However, I have no idea what it is I am looking for, and being honest, I dont really know what a codec is at all.