The main Operating System
that runs on most consumer and workstation computers, if some flavor of UNIX
isn't being used. It was developed by Microsoft back in The Eighties
to provide a graphical user interface to DOS and it was to help IBM PC based computers
at the time to compete against Apple and Amiga, who already shipped with their own graphical interfaces. 30 years later, Windows is the most widely used operating system on personal computers, commanding over 90% of the market share. The main reason why Windows has been so prevalent (cut-throat tactics in the late 1990s notwithstanding) is because of its degree of compatibility with older software. This 
demonstrates not only its upgradability, but said compatibility.
The Early Versions, Windows 1.0 and 2.x
Released in 1985, the first version of Windows wasn't really considered a full blown operating system, lacking features like memory management. But it did support basic multitasking and introduced API standards that allow programs that run in Windows 1.0 to work all the way to the latest version with little tweaking or minor recompiling if necessary. While it did have windows to display applications in, due to patents held by Apple, Windows 1.0 could not have overlapping windows. Needless to say, the first version wasn't very successful.
Windows 2.0 was released two years later, which improved on the graphical interfaces, such as allowing overlapping windows and minimizing and maximizing them. A plethora of apps came with the OS, including a calender, contacts, and Reversi. It also supported the new VGA standard, but only 16 colors were available. Windows 2.0 sold much better than the first version, though software developers still maintained DOS versions of their programs. Windows 2.0 was also heavily dependent on DOS still to manage hardware resources, including a memory access limitation of 1 megabyte.
Of course, the use of patented overlapping windows caught the attention of Apple, and they filed a lawsuit in 1994 over it.
Windows 3.0 and 3.1
Windows 3.x is where Windows really took off. 3.0 was released in 1990 with the design built to take advantage of protected memory support as well as better driver support. This allowed programs to behave better when run simultaneously as well as access more memory. The user interface was given many improvements as well. A year later, Microsoft added multimedia extensions to take advantage CD players being available to computers.
The only issue with 3.0 was that it was fairly crash prone, as the minimum requirement was the 8086 processor, which lacked certain key features. In 1992, Windows 3.1 was released, requiring at the minimum a 286 processor and 1MB of RAM. This helped increase stability quite a bit. Other features included support for the new TrueType Font standard, multimedia playback capabilities, and basic networking. User interface improvements included the ability to drag and drop icons, for instance, dragging a word document to a printer and having it printed out.
Despite all of the improvements, many games at the time still used DOS for its more 'direct' ways of accessing resources, especially since there was no API in Windows yet.
Microsoft Goes Pro: OS/2 and Windows NT
In 1988, Microsoft and IBM started working on a workstation centric operating system later dubbed OS/2. It was an ambitious effort to allow programmers ease of developing software so that, for instance, the program no longer had to go through BIOS to access hardware, it could call it from the OS. It was also designed to be multitasking, much like UNIX.
However, the relationship ended in 1990. Windows 3.0 took off and sold millions, mostly because it was bundled with computers while OS/2 was an expensive upgrade. Microsoft also favored open hardware platforms while IBM wanted to restrict OS/2 to just their computers. But Microsoft took all of these ideas that were good from OS/2, took all the ideas that were good from Windows 3.0, and started from scratch to build a new operating system kernel: Windows NT.
Windows NT was designed from the ground up with the prospect of being portable
to other processor architectures of the time, secure and more stable than the Windows that ran on top of MS-DOS, and networking. Other features included the concept of security levels for users (so that a user cannot crash the operating system) and compatibility with OS/2 and previous Windows applications.
Microsoft didn't intend for Windows NT to replace Windows 3.x, instead it was a companion product for those with high end systems who wanted the features, namely at the professional sector.
The Menu to Rule Them All: Windows 95, 98, and Me
Microsoft continued with their consumer versions of Windows by being on top of MS-DOS. The next version was Windows 95, introducing the one noticeable feature that set the trend for a lot of other desktop based interfaces: the Start Menu. While initially confusing for some users at first, the Start Menu was where most of the applications and settings could be found and accessed. Of course, there were other technical improvements as well, such as being 32-bit, supporting plug-and-play hardware, preemptive multitasking, and increased stability.
Windows 98, released three years later, added improvements to hardware support and internet connectivity. However, the MS-DOS based Windows came to a crash following the release of Windows Millenium Edition, or ME. It was buggy, unstable, and while it did introduce some neat features (such as System Restore), many people hated it and stuck with Windows 98.
Aside from that, Windows 9x and ME had its share of problems. The world became familiar with the Blue Screen of Death, a fatal system error which was a very common occurence. Infamously during a live demonstration of Windows 98, the OS crashed while attempting to show off its plug and play features
, to which Bill Gates quipped "That must be why we're not shipping Windows 98 yet"... after applause and cheering
. Much of this could be attributed to how Windows did backwards compatibility called DLL Hell
Windows 2000 and XP, Keeping the Crown
Windows NT continued developments alongside Windows 95, 98, and ME. As it was not dependent on DOS, it enjoyed more stability and security. Windows 2000 introduced a laundry list of features, including some from Windows 98, and improved hardware support for USB devices. It also was the first Windows to protect key files from being overwritten. However, security was an issue as the internet grew in this period and hackers found security holes to exploit the OS. Despite all this, those who've used Windows 2000 regularly claimed it was the best Windows OS, even after the release of the next version, Windows XP.
Windows XP was the first NT version of Windows with consumer support with the Home version. It also had a Professional version for businesses and the like. XP rapidly rose to usage after Service Pack 1 was released, mostly due to increased stability (it was practically immune to DLL Hell) and compatibility with previous Windows programs. Key changes were also made to the user interface, such as the Start Menu and Windows Explorer.
Not everything was sunshine and roses, though. Because of its popularity, it was a common target for computer viruses. It didn't help that few people understood the concept of privileged user accounts. By default, Windows XP user accounts are administrators, which means that a program that runs has free reign on all OS resources. These issues were primarily used as ammo by Apple to gain popularity of its "virus immune" OS.
Microsoft Stumbles, Then Picks Up the Slack - Windows Vista and 7
Vista had a long history, starting development all the way back in 2002. Microsoft wanted to implement ambitious features for the time, including a relational database-like file system WinFS and a radically new API for the interface. Security was also a primary concern of Microsoft, so that was kept in mind as well. However, feature creep kept delaying the operating system over and over again, and features had to be dropped. It didn't help that Microsoft started to look outdated with the release of Mac OS X 10.4 in 2005. Years too late, Microsoft finally released the new OS as Windows Vista. It was bloated, required more resources than the average computer had at the time to run smoothly, and one of the touted new security features, User Account Control, was seen as annoying and was a key reason for some to skip the OS.
Of course, Microsoft wasn't going to let Apple take the spotlight and quickly reworked and optimized Windows Vista into Windows 7. Windows 7 was well received, being referred to as "what Windows Vista should've been". While Windows 7 was only an incremental improvement on the list of features available from Windows Vista, the user interface was improved and by the time it was released, the average computer could actually run it smoothly.
Microsoft Goes Metro - Windows 8
A few years prior to 2012, Microsoft had a few portable media players called Zune. Though it didn't put a dent in Apple's dominance with the iPod, it had a notable feature that turned heads: its user interface. Dubbed Metro (after being inspired by signs from Seattle's metropolitan transit system), it focused on clean, flat topology with emphasis on being efficient, using the actual content as UI elements. A lot of publications felt that this was a good competitor against Apple, to which even Apple started to take cues and started suing companies who were allegedly copying their iOS design.
With the rise of tablets and other portable devices, Microsoft felt it was necessary to unify the user interface so that it was consistent across any Windows powered device. Windows 8 adopted principles of Metro and replaced the Start Menu with the Start Screen. This screen filled up the display with an array of icons, some with apps that showed relevant content (such as a weather app icon showing... well, the weather).
Despite the positive reaction from Metro, there was much criticism regarding using it for desktop based environments and it was clear that the language used in Windows 8 was tailored to touch devices, not keyboards and mice. While Windows 8 does improve on Windows 7 in underlying areas, most people criticized the Start Screen and used it as a reason to avoid Windows 8, even calling it the next Vista. However, Microsoft doesn't seem too keen on going back (but there are 3rd party add-ons to bring back the Start Menu).