Windows is a graphical user interface and enhancement to DOS from Microsoft Corp. Some claims it is an operting system, others claims it is not. Actualy some claims that DOS is not an operating system - it is an advanced interupt handler with some utility programs (it can not handle memory between programs, timeshare programs, schedule programs, it can not really handle devices)
Nowadays Windows is a collection of operating systems from Microsoft. Starting with windows 1.0 and windows 2.0 which was not that usable at all. First with Windows 3.0 became users attracted to Windows. 3.0 was succeeded by 3.1 which added some common dialogboxes which could be used by all programs. This made programs look and behavie in the same way, making it easier to learn and to use the programs.
With Windows for Workgroups Microsoft added networking features to Windows. The last 16-bit windows was WfW 3.11, which had matured to include 32-bit Windows subsystem (to run some 32-bit Windows programs), advanced file-io drivers, virual memory, disk chache and more.
The first 32-bit version of Windows was MS Windows NT 3.0. It was later replaced of 3.5 and 3.51. Like OS/2 it was released in Workstation, Server and Advanced Server versions.
In late 1995 the first 32-bit Windows for use at homes was relaesed. It's name was Windows95. It was not a true 32-bit operating system since central parts was still 16-bit. At first it does not sound to matter if it is 32-bit or 16-bit. But knowing that the processor is 32-bit and to use it in 16-bit mode requires a lot work from the processor, many hated Microsoft for this. Actually Microsoft released a Windows95 in 1996 with more parts being 32-bit. This was never pushed by Microsoft. Doing that would only make people believe that they had bought an incomplete operating system. Later Microsoft released Windows95 OSR 1, OSR 2, OSR 2.5.
The greatest change for most users was that the user interface had changed. The old Program manager was replaced by Explorer. This was later called the Explorer interfce. But many other features was added. For example modems was treated as resources with their own drivers. This means that the operating systems keeps a logical layer around the resource presenting a common interface for other programs to use. Before, all programs had their own drivers to every modem. Now there was only need for one driver for each modem - for Windows95.
Another change was that network drivers did not use the first 640 KB that had been so limiting before. And the networking worked a lot better. Both more stable and faster.
The first version of WindowsNT to get Explorer interface was WindowsNT 4.0. During the time WindowsNT 4.0 was released there was a major competition of which web-server that would take the bigest market share. Microsoft almost killed the market when they bundled WindowsNT Server with their web-server IIS and limited the network connections in WindowsNT Workstation. They also bundled their web-browser Internet Explorer with WindowsNT and Windows95. Doing this almost killed the marked for web-browsers.
Many features that was promised to come with WindowsNT 4.0 was missing when it was released. Many featues has been added with the Service Packs (SP) Microsoft has released. 6 SPs was released before next version came - Windows2000.
Before Windows2000 was released, two more Windows was relased. In 1998 Windows98 and later Windows98 Second Edition was released. It looked like Windows95, it behaved like Windows95 - it was Windows95 (?) with a lot of bugfixes and small enhancements.
Windows98 is not the last incarnation of Windows95. There is a Windows ME (Millenium Edition). Since we have past the change of millenium the name might be changed when it is finally released.
Windows have a litle brother for small computers WindowsCE. It has almost the same interface as Windows95 but limited. One limitation is that is hard to quit a program. You just open them and they will keep running. WindowsCE has not been very successful, at least not in Europe. It has now been replaced of WindowsPC.
There are also crippled versions of WindowsNT/2000 which is ment to be used in systems like ATMs.