Microsoft Windows is the most widely used Operating System for desktop PCs and laptops with a total market share of over 92% worldwide. It is also used as a server OS where it holds around half the market share.
Windows does not describe a single operating system but is used as a blanket term for a number of different systems and versions. You can find more information about each version with the links below.
DOS Shell 
These versions of Windows were all 16 bit applications that ran on top of the DOS operating system as a shell. At this point, Windows was only considered an Operating System because it improved some OS specific functions that DOS could not handle or added some that DOS did not support.
DOS Integrated 
The introduction of Windows 95 brought 32 bit and multi threaded GUI applications to users along with a better file system. Windows 95 through ME did have a number of problems with reliability and speed that stemmed from the 16 bit GUI and the hybrid 16/32 bit nature of the OS.
NT Based 
The NT versions of Windows have proven more stable than the DOS Integrated versions with a better FileSystem and full 32 bit processing. After Windows ME, Microsoft integrated the Home based OS line with their NT line of Windows OS's into Windows 2000. Most recently, new versions of Windows XP have become available with 64 bit software as well as the newest OS in the Windows line, Windows Vista, which also supports 64 bit software.