Typically, overlooking firmware hacks etc., the included DVD player app will only play standard DVDs encoded to region 0, or the region of your built-in drive, as well as Video_ts folders.
Quicktime is used for standard mpg, mov etc. files. It can also play divx,xvid once the plug-in is installed.
Better yet, do what most of us do, and download vlc and mplayer. They're the two all purpose swiss-army knife media players. What one won't play, chances are the other will.