I'm going to buy a disk drive, and different brands of drives have a different DOS. What exactly is DOS?
DOS (usually pronounced to rhyme with "moss") stands for Disk Operating System. Basically, this is a program which allows the computer to work with a disk drive. On most computers, DOS lets you save and load files, view disk directories (lists of files stored on disks), rename files, erase files, copy files from one disk to another, copy entire disks, format blank disks (prepare them for use), and other functions.
There are many different types of DOS for different computers, and they're usually incompatible with each other. It's important that you use the proper DOS for your computer, disk drive, and system configuration. Fortunately, most disk drives (or computers with built-in disk drives) already include the proper DOS.
Usually DOS comes on a disk that must be inserted in the disk drive before you turn on the computer. It loads automatically when the power is switched on. This process is called booting up. An exception is Commodore DOS, which is stored in Read Only Memory (ROM) chips within the disk drive itself. Commodore DOS is available whenever the computer and disk drive are powered up.
There are many versions of DOS even for the same computer. As revisions, corrections, and updates are made, new versions of DOS are released, usually denoted by different numbers. Examples are Atari DOS 1 (the original version), DOS 2.0S (improved single-density), and DOS 3 (enhanced density); PC-DOS 1.1 (the original version), DOS 2.0 (with improvements added for hard disks), and DOS 2.1 (modified for the PCjr); Apple DOS 3.3 (originally intended for the Apple II and II+) and ProDOS (introduced with the Apple IIe and IIc); and so on. Commodore DOS is harder to modify since it's embedded in ROM chips, but unofficial updates are usually made when new models of disk drives are introduced.
In addition to the DOS versions released by computer manufacturers, there are also custom versions of DOS sold by independent companies for certain computers. Examples are OS/A+ DOS for Atari computers, CP/M-86 for IBM computers, and CP/M-80 for numerous personal computers. Sometimes a custom DOS is compatible with the manufacturer's DOS, and sometimes it requires extra hardware (such as a CP/M board).
The disk drive you buy for your computer will probably come with the right DOS for your system. If it doesn't, the dealer can recommend the proper DOS or a compatible custom DOS.