Boot disk

A boot disk is a removable digital data storage medium from which a computer can load and run (boot) an operating system or utility program. The computer must have a built-in program which will load and execute a program from a boot disk meeting certain standards.
While almost all modern computers can boot from a hard drive containing the operating system and other software, they would not normally be called boot disks (because they are not removable media). CD-ROMs are the most common forms of media used, but other media, such as magnetic or paper tape drives, ZIP drives, and more recently USB flash drives can be used. The computer's BIOS must support booting from the device in question.
One can make one's own "boot disk" (typically done to prepare for when the system won't start properly).
Uses.
Boot disks are used for:
Process.
The term "boot" comes from the idea of lifting oneself by one's own bootstraps: the computer contains a tiny program (bootstrap loader) which will load and run a program found on a boot device. This program may itself be a small program designed to load a larger and more capable program, i.e., the full operating system. To enable booting without the requirement either for a mass storage device or to write to the boot medium, it is usual for the boot program to use some system RAM as a RAM disk for temporary file storage.
As an example, any computer compatible with the IBM PC is able with built-in software to load the contents of the first 512 bytes of a floppy and to execute it if it is a viable program; boot floppies have a very simple loader program in these bytes. The process is vulnerable to abuse; data floppies could have a virus written to their first sector which silently infects the host computer if switched on with the disk in the drive.
Media.
Bootable floppy disks ("boot floppies") for PCs usually contain DOS or miniature versions of Linux. The most commonly available floppy disk can hold only 1.4 MB of data in its standard format, making it impractical for loading large operating systems. The use of boot floppies is in decline, due to the availability of other higher-capacity options, such as CD-ROMs or USB flash drives.
Device selection.
A modern PC is configured to attempt to boot from various devices in a certain order. If a computer is not booting from the device desired, such as the floppy drive, the user may have to enter the BIOS Setup function by pressing a special key when the computer is first turned on (such as , , , or ), and then changing the boot order. More recent BIOSes permit the interruption of the final stage of the boot process and invoke the Boot Menu by pressing a function key (usually or ). This results in a list of bootable devices being presented, from which a selection may be made.
Modern Apple Macs will display the Boot Menu if user presses the or while the machine is starting.
Requirements.
Different operating systems use different boot disk contents. All boot disks must be compatible with the computer they are designed for.
All files must be for the same version of the operating system. Complete boot disks can be prepared in one operation by an installed operating system; details vary.