You can use Qemu to boot a CD ISO image you downloaded before you even burn it. This could be useful for just checking it out before you decide to spend a CD for burning it or to take screenshots you otherwise wouldn't be able to from the real bootup.

Here's a simple command to do it:

qemu -cdrom cdimage.iso

You can even install a CD image on a special virtual disk you can create on qemu. This disk is actually not a disk, of course, but a disk image which contains free space that can be used within qemu emulation.
To create this disk image do this:

qemu-img create myimage.img mysize

Of course, replace myimage.img with the desired name and "mysize" with the desired amount of free space you want it to take, for example 2GB.

To be able to install from a CD image, you have to boot it with the support of the qemu disk image you created above. Here's the command for that:

qemu -cdrom cdimage.iso -boot d /path/to/diskimage.img

cdimage.iso is the name of the CD ISO you need to boot and diskimage.img is the disk image containing free space you can install on (from the CD ISO you booted from).

There are many ways you can use qemu. For more information you can type man qemu on your GNU/Linux terminal or read Qemu documentation.