In the old days we all had CD-drives, but most modern computers these days don't have an optical drive. This makes things even more efficient, because you need to think smart. Some people choose to have an external optical drive, but why all the hassle when you can have a bootable USB or a bootable partition on your harddrive. There ar many guides on the internet, how to get that done manually. Again, why the hasse when we have UNetbootin, the perfect software for booting your favorite ISO's from your harddrive!