Qemu is a x86 Emulator/Virtual Machine.
Grabbing debug messages:
By default debug messages from ReactOS are sent to the serial port (COM1). For grabbing that output, you need at least qemu version 0.6.1 where you can specify the -serial option. Like this: $ qemu -serial stdio -cdrom roslive.iso -boot d
If you are testing the setup cd, note that you can change the kernel arguments by editing the txtsetup.sif file (so things actually get written to the serial port).