Why can't I read a rmvb video file download?

[California]

I have never downloaded any video or even bought iTunes before (!). But I just caved in because I wanted to see Moore's film. What is a rmvb file and why can't I open it on my stock PPC 1.5 mac mini G4?

 

[Schroedinger]

http://www.jakeludington.com/dv_hacks/20051210_how_to_play_rmvb_files.html