With Nero Burning ROM you can create a bootable disc with which the computer can be started without having to access the hard drive. For this reason a bootable disc is often used as an “emergency disc” to start the computer if it is not possible to access the hard drive.
Bootable discs are created in accordance with the “El Torito” standard, an extension to the ISO-9660 standard, which defines the structure of data discs. The disc contains a boot image and an ISO part. The boot image contains all files that are required to load the operating system and to start the computer. The ISO part can contain any number of data files that you can back up using this method.  

Bootable Disc