A disk operating system (DOS) will load from a floppy disk each time a computer starts, and will access that disk for software to complete operations. MS-DOS is probably the most well-known DOS, and was purchased by Microsoft Corporation from a system known as QDOS. As operating systems became more complicated and took up more space, they began to be permanently installed on hard drives, which are faster and more reliable than floppy disks, and can store more data. This was encouraged by a steady drop in hard drive prices.
DOSes are usually text-based and without graphics to save space on a floppy disk. Although DOSes have mostly been replaced by Windows, Macintosh, and Linux systems, among others (sporting a GUI, or Graphical User Interface), they are still in use in some places, mostly on older computers. This may be because of a lack of money, the need to use software which still runs on a DOS system, nostalgia for an older operating system, or the belief that text-based systems are more efficient.
Although GUIs are more popular, in times of emergency or when one needs to install special software, a boot disk allows a user to start his computer into a DOS.