NT Workstation is no longer supported by Microsoft as of July 1st, 2003 and NT Server is no longer to be supported as of January 1st, 2004. While this does not mean the OS is obsolete, it does prevent users from getting assistance from the maker.
Released in 1996, Microsoft's attempt at designing a fully 32-bit operating system to take advantage of the new 386 processor from Intel. The kernel effort was headed by David Cuttler, a developer who had also worked on DEC's VMS operating system, a tried-and-true server OS that pre-dated UNIX. As well as trying to build a well-designed OS, Microsoft wanted to have as much compatibility with the Windows 3.1 API as possible. In this they mostly succeeded - NT 3.1 was fairly compatible with Windows applications, but had no real support for MS-DOS applications.
Windows NT was built on a microkernel architecture. It used a so-called Hardware Abstraction Layer (HAL) to allow many device drivers to be dynamically loaded and unloaded. As a result, it initially ran not just on Intel chips, but also on hardware from other manufacturers including DEC and HP. Even today it remains fairly modular, barring the one big change that happened with NT 4.0 - moving the GUI primitives from user-level to kernel-level. This was a controversial move that meant that if any bugs occurred in the graphical system, they would cause a kernel error (and usually a complete system crash). It was done to increase performance, and reflects Microsoft's emphasis on providing its users with a sleek and intuitive user interface, even at the expense of stability. Windows NT 4.0 ran on x86, Alpha, MIPS, and PowerPC
Windows 2000 followed Windows NT 4.0, and up to and including that release the NT branch was considered only really useful for servers and office workstations. Multimedia and gaming features were always far behind the Windows 95 family. Windows 2000 also spelled the end of support for processors other than Intel, as their popularity was very low.
Finally, with the release of Windows XP Microsoft managed to migrate home users away from the MS-DOS-based operating systems and built a responsive and multimedia-enriched version of Windows NT for the end user. Today most companies have moved on from Windows NT and use Windows 2000, Windows XP or Windows Server 2003 on their servers and workstations.