A boot disk is actually not a computer disk in the shape of a boot. If it was, most disk drives would have a difficult time reading it. Instead, a boot disk is a disk that a computer can start up or "boot" from. The most common type of boot disk is an internal hard drive, which most computers use to start up from. The operating system installed on the hard drive is loaded during the boot process.
However, most computers allow you to boot from other disks, including external Firewire hard drives, CD-ROMs, DVD-ROMs, and floppy disks. In order to function as boot disks, these disks need to have an operating system installed that is understandable by the computer. This can either be a full-blown operating system like Windows or Mac OS X, or a small utility operating system, such as Norton Utilities or DiskWarrior.
CD and DVD boot disks are often used to start up a computer when the operating system on the internal hard drive won't load. This can happen when bad data blocks or other errors occur on the disk. By running a disk repair utility from the CD or DVD, you can often fix the hard drive and restart from it, using the full operating system.