Originally Posted by
Splash
The install media is what it says, the media you use to install the OS. The other 2 are "Live CDs", ie you put the disc in your drive and boot from it so you can play with the OS in a non-destructive way (also can be handy for system rescue) - I'm guessing the "Desktop Live Media" is a GNOME Window Manager, as opposed to the KDE version which is self-explanatory.
The install media you select depends on your architecture - is it i386, x86_64 (64 bit) or PPC PowerPC, essentially for older Macs) that you need?