Non-nerd's Guide to Computers/win
Windows in current versions is a family of operating systems from Microsoft. Most recent is Windows 10, and an old discontinued example in the family is Windows XP (other discontinued mentioned below).
Historically, Windows was a graphical interface run on top of DOS developed by Microsoft. It later was merged with the Windows NT line and became a "complete" operating system. By comparison it has a large market share on desktop PCs, but has been losing its supremacy due to the improvements made to alternative free OSs, where some even have the capacity to run applications meant only for Windows. Another factor in the decline of Windows as a desktop OS has been the increased use of resources (memory, CPU and hard drive space) and that later versions haven't catered to the real needs of users. This was made mostly evident by the Windows ME (Millennium Edition) and later with Windows Vista, where a merely mercantile approach seems to drive each new release and not technological innovation.