MS-DOS (short for Microsoft Disk Operating System) is an operating system commercialized by Microsoft. It was the most commonly used member of the DOS family of operating systems and was the dominant operating system for the PC compatible platform during the 1980s. It has gradually been replaced on consumer desktop computers by various generations of the Windows operating system.
MS-DOS was originally released in 1981 and had eight major versions released before Microsoft stopped development in 2000. It was the key product in Microsoft's growth from a programming languages company to a diverse software development firm, providing the company with essential revenue and marketing resources.

History
Adapted from original source: PC Museum

MS-DOS 1.14 - July 1981 - Microsoft rebranded 86-DOS as MS-DOS in July 1981, having bought the rights from SCP.
MS-DOS 1.25 - May 1982 - first release for IBM PC compatibles marketed under different brands (COMMAND.COM is 4986 bytes)
MS-DOS 2.0 - March 1983 - support for PC XT: introduced subdirectories, handle-based file operations, command input/output redirection, and pipes. Microsoft decided to use backslashes as pathname separators rather than slashes as on Unix apparently due to the latter character being used as the switch character in most DOS and CP/M programs. Adds support for hard drives and 360KB floppy disks
PC DOS 2.1 - October 1983 - support for IBM PCjr
MS-DOS 2.11 - March 1984 - non-English language and date format support (COMMAND.COM is 16229 bytes)
MS-DOS 2.25 - October 1985 - better support for Japanese Kanji, and Korean character sets, shipped to western Pacific countries only
MS-DOS 3.0 - August 1984 - added support for PC AT: 1.2 MB floppy disks and hard disk partitions of up to 32MB, one primary and one "logical drive" in an "extended partition"
MS-DOS 3.1 - November 1984 - support for Microsoft networking
MS-DOS 3.2 - January 1986 - support for 3.5 inch, 720 kB floppy disk drives (v 3.21 COMMAND.COM is 23612 bytes)
PC DOS 3.3 - April 1987 - support for IBM PS/2: 1.44 MB floppy disk drives, added codepage support (international character sets) (COMMAND.COM is 25307 bytes)
MS-DOS 3.3 - August 1987 - supported multiple logical drives (COMMAND.COM is 25276 bytes)
MS-DOS 4.0 - June 1988 - derived from IBM's codebase rather than Microsoft's
PC DOS 4.0 - July 1988 - added DOS Shell & support for hard disks of >32MB using the format from Compaq DOS 3.31. But it had many bugs and less free conventional memory than before. Generally regarded as an unpopular release
MS-DOS 4.01 - December 1988 - bug-fix release (COMMAND.COM is 37557 bytes)
MS-DOS 5.0 - June 1991 - memory management, full-screen editor, QBasic programming language, online help, DOS Shell task switcher, and FastLynx file transfer utility licensed from Rupp Technology. Also used as the basis for Virtual DOS Machine for Windows NT4 through Vista. (COMMAND.COM is 47845 bytes)
MS-DOS 6.0 - March 1993 - added DoubleSpace disk compression, disk defragmentation, and other features (COMMAND.COM is 52925 bytes)
MS-DOS 6.2 - November 1993 - bug fix release (COMMAND.COM is 54619 bytes)
MS-DOS 6.21 - February, 1994 - following Stac Electronics lawsuit, removed DoubleSpace disk compression (COMMAND.COM is 54619 bytes)
PC DOS 6.3 - April 1994
MS-DOS 6.22 - June 1994 - last official stand-alone version. DoubleSpace replaced with non-infringing but compatible DriveSpace tool (COMMAND.COM is 54645 bytes)
PC DOS 7.0 - April, 1995 - bundles Stacker in place of DriveSpace
MS-DOS 7.0 - August 1995 - shipped embedded in Windows 95. Included Logical block addressing and Long File Name (LFN) support (COMMAND.COM is 92870 bytes)
MS-DOS 7.1 - August 1996 - shipped embedded in Windows 95B (OSR2) (and Windows 98 first and second editions in June 1998 and May 1999). Added support for FAT32 file system (COMMAND.COM is 93812, 93880 or 93890 bytes in 95B, 98 or 98SE respectively)
MS-DOS 8.0 - September 2000 - shipped embedded in Windows Me. A subset is included with 32-bit versions of Windows XP and Windows Vista. Last version of MS-DOS. Removes SYS command, ability to boot to command line and other features (COMMAND.COM is 93040 bytes)
PC DOS 2000 - year 2000-compliant version with minor additional features. Final member of the MS-DOS family Versions and release dates

Features
MS-DOS employs a command line interface and a batch scripting facility via its command interpreter, COMMAND.COM. MS-DOS was designed so users could easily substitute a different command line interpreter, for example 4DOS.
Beginning with version 4.0, MS-DOS included DOS Shell, a file manager program with a quasi-graphical text user interface (TUI) that featured menus, split windows, color themes, mouse support and program shortcuts using character mode graphics.

User interface
MS-DOS was not designed to be a multi-user or multitasking operating system, but many attempts were made to add these capabilities. Terminate and Stay Resident (TSR) system calls were originally designed for device drivers and extensible plugins that enhanced or added features. Companies such as Borland began to tap into the TSR design with products like SideKick. Add-on environments like TopView and especially DESQview attempted to provide multitasking, and achieved some success when later combined with the virtual 8086 mode and virtual memory features of the Intel 80386 and later processors.

Multitasking
On the IBM PC (and clones) platform, the initial competition to the PC-DOS/MS-DOS line came from Digital Research, whose CP/M operating system had inspired MS-DOS. Digital Research developed CP/M-86 and offered it to computer manufacturers as an alternate to MS-DOS and Microsoft's licensing requirements.
In the business world, the PC platform that MS-DOS was tied to faced competition from the Unix operating system which ran on many different hardware architectures. Microsoft even sold a version of Unix called Xenix.
In the emerging world of home users, a variety of other hardware platforms were in serious competition with the IBM PC: the Apple II, early Apple Macintosh, the Commodore 64 and others. At first, the competition for these other platforms was with IBM PC computers running MS-DOS. With the advent of IBM PC clones all running on Intel processors, the name IBM became less important to home users. What was important was keeping up with Intel's steadily increasing clock speeds and the ability to run MS-DOS.
Microsoft and IBM together began what was intended as the follow-on to DOS, called OS/2. When OS/2 was released in 1987, Microsoft began an ad campaign announcing that "DOS is Dead", boldly proclaiming version 4 was the last full release.
MS-DOS had grown in spurts, with many significant features being taken (or duplicated) from other products and operating systems, as well as reverse-engineering tools and utilities including Norton Utilities, PC Tools (Microsoft Anti-Virus), QEMM expanded memory manager, DOS/4GW (a 32-bit DOS extender), Stacker disk compression, and so on. The advent of OS/2, which offered a number of advanced features which had been written together, was seen as the legitimate heir to the "kludgy" DOS platform.
Digital Research, recognizing the need to continue the lower-level platform represented by DOS, then developed DR DOS 5, which mirrored the OS/2 "platform integration" model by adding features which were available only as third-party add-ons for MS-DOS. Unwilling to lose any portion of the market, Microsoft responded by announcing the "pending" release of MS-DOS 5.0 in May of 1990. This effectively killed most DR DOS sales, until the actual release of MS-DOS 5.0 in June 1991. Digital Research brought out DR DOS 6, which sold well until the "pre-announcement" of MS-DOS 6.0 again stifled the sales of DR DOS.
Microsoft has been accused of carefully orchestrating leaks about future versions of MS-DOS in an attempt to create what in the industry is called FUD (fear, uncertainty, and doubt) regarding DR DOS. For example, in October 1990, shortly after the release of DR DOS 5.0, and long before the eventual June 1991 release of MS-DOS 5.0, stories on feature enhancements in MS-DOS started to appear in InfoWorld and PC Week. Brad Silverberg, Vice President of Systems Software at Microsoft and General Manager of its Windows and MS-DOS Business Unit, wrote a forceful letter to PC Week (November 5, 1990), denying that Microsoft was engaged in FUD tactics ("to serve our customers better, we decided to be more forthcoming about version 5.0") and denying that Microsoft cops features from DR DOS: "The feature enhancements of MS-DOS version 5.0 were decided and development was begun long before we heard about DR DOS 5.0. There will be some similar features. With 50 million MS-DOS users, it shouldn't be surprising that DRI has heard some of the same requests from customers that we have." — (Schulman et al. 1994).
The pact between Microsoft and IBM to promote OS/2 began to fall apart in 1990 when Windows 3.0 became a marketplace success. Much of Microsoft's further contributions to OS/2 also went in to creating a third GUI replacement for DOS, Windows NT.
IBM, which had already been developing the next version of OS/2, carried on development of the platform without Microsoft and sold it as the alternative to DOS and Windows.

End of MS-DOS
As a response to Digital Research's DR-DOS 6.0, which bundled SuperStor disk compression, Microsoft opened negotiations with Stac Electronics, vendor of the most popular DOS disk compression tool, Stacker. In the due diligence process, Stac engineers had shown Microsoft some Stacker source code. Stac was unwilling to meet Microsoft's terms for licensing Stacker and withdrew from the negotiations. Microsoft chose to license Vertisoft's DoubleDisk, using it as the core for its DoubleSpace disk compression. Legal issues
Main article: Windows NT
Windows NT, although not based on DOS, provides a command-line interface similar to MS-DOS's character-mode interface. This command line is provided by a native executable, cmd.exe. Many command-line applications (known as console applications) for Windows are incorrectly referred to as DOS applications, when actually they are full Windows applications which use the console for their output rather than a graphical interface, and cannot be run under any version of MS-DOS.
Windows NT can run MS-DOS programs through the use of the NTVDM (NT Virtual DOS Machine), and the 16-bit command.com interpreter from MS-DOS 5.0 is still included to maintain application compatibility with programs that expect it (This is illustrated by the output produced by the command "command.com /k ver", which displays "MS-DOS Version 5.00.500" in the console window). The command "ver" returns the string "Microsoft(R) Windows DOS" when executed under command.com, but "Microsoft Windows XP [Version 5.1.2600]" (or similar depending on the version of NT) when run from cmd.exe.
Recent versions of NT for x64 architectures, including Windows XP Professional x64 Edition, Windows Server 2003 x64 and Windows Vista x64, no longer include the NTVDM and can therefore no longer natively run MS-DOS (or 16-bit Windows) applications. For MS-DOS and Windows 3.11 or earlier programs, however, there exist alternatives in the form of emulators such as Microsoft's own Virtual PC, VMWare, Bochs, DOSBox, etc.

Windows NT
From 1983 onwards, various companies have worked on graphical user interfaces (GUIs) capable of running on PC hardware. With DOS being the dominant operating system several companies released alternate shells, e.g. Microsoft Word for DOS, XTree, and the Norton Shell. However, this required duplication of effort and did not provide much consistency in interface design (even between products from the same company).
Later, in 1985, Microsoft Windows was released as Microsoft's first attempt at providing a consistent user interface (for applications). The early versions of Windows ran on top of MS-DOS and its clones. At first Windows met with little success, but this was also true for most other companies' efforts as well, for example GEM. After version 3.0 (1990), Windows gained marked acceptance.
Later versions (Windows 95, Windows 98 and Windows Me) used the DOS boot process to launch itself into protected mode. Basic features related to the file system, such as long file names, were only available to DOS when running as a subsystem of Windows. Windows NT ran independently of DOS but included a DOS subsystem so applications could run in a virtual machine under the new OS. With the latest Windows releases, even dual-booting MS-DOS is problematic as DOS may not be able to read the basic file system.

Legacy compatibility
Several similar products were produced by other companies. In the case of PC-DOS and DR-DOS, it is common but incorrect to call these "clones". Given that Microsoft manufactured PC-DOS for IBM, PC-DOS and MS-DOS were (to continue the genetic analogy) "identical twins" that diverged only in adulthood and eventually became quite different products; DR-DOS was a clone of itself once removed.
These products are collectively referred to as DOS. However, MS-DOS can be a generic reference to DOS on IBM-PC compatible

PC-DOS
DR-DOS, Novell DOS, and OpenDOS
FreeDOS, FreeDOS 32, and GNU/DOS
PTS-DOS
DOSBox, a popular MS-DOS emulator See also
"IBM wanted CP/M prompts. It made me throw up." -- Tim Paterson