Although the official names are Windows 2000/XP/Vista/7/8/8.1, the Windows NT "version" of these releases are 5.0 (Windows 2000), 5.1 (Windows XP), 6.0 (Windows Vista), 6.1 (Windows 7), and 6.2/6.3 (Windows 8/8.1). Windows NT is no longer a part of the actual product names, and is indeed only used to refer/identify operating systems internally.
This table on Wikipedia helps to summarize the Windows NT version of various Windows releases. Each OS is based off of Windows NT, where Windows 2000 is the first release to drop NT from the product name. Note that the Windows (kernel) version convention, is changing with the release of Windows 10, where the version number will jump from 6.3 (Windows 8.1) to 10.0 (Windows 10), breaking the conventional versioning/numbering scheme for the Windows NT-family.
Internally, as there were two "branches" of Windows development - one following the Win9x/DOS system (16 and 32-bit), and one following the WinNT system (true 32-bit, and later, 64-bit), the eventual goal was to merge both into a unified codebase. The architecture of Win9x made it a difficult task; although both implemented the Win32 API, the actual merger of the NT (commercial) and 9x (consumer) product lines was delayed until the release of Windows XP.
