Windows NT series use the same user interfaces of that of the Windows series. The user interface of Windows NT 4.0 is like that of Windows 95. The user interface of Windows 2000 is like that of Windows 98 and Windows Me. This doesn't mean NT series are like Windows series, because many aspects of them are different.
Let's look into Windows NT 4.0 and Windows 2000 as examples. Windows NT Workstation 4.0 and Windows 2000 Professional are dedicated to commercial users. They are designed to work on computer workstations. This means they may be used by multiple persons, one at a time locally, and simultaneously remotely. Because of the design, these versions of Windows NT support multiple users once you finish installing them. Windows NT Server 4.0 and Windows 2000 Server Family products are designed for servers so that they support not only multiple users, but also computer servers. They are also automatically configured to support multiple users once they are installed, and they allow remote file shares accessed by multiple users.
Furthermore, Windows 2000 has three Server editions: Server, Advanced Server and Datacenter Server. Terminal Services can be installed on Windows 2000 Server editions. Terminal Services support multiple users, too, and because they allow desktop control, they provide much higher capability than multi-user remote file shares.
Because all of them support multiple users initially, they are quite different from Windows 9x series from the first point of view. Once you installed one of them, you would have to configure it to allow proper users to access them.
Under Windows NT, 16-bit programs are always run under NTVDM (Windows NT Virtual DOS Machine). Under Windows 9x, 16-bit DOS programs are run under WinOldAp.exe, and 16-bit Windows programs are run under WOWExec.exe (Windows On Windows). The most obvious difference is, 16-bit Windows programs can hang a Windows 9x system up but they cannot hang a Windows NT system up. The cause of this difference is: Windows 9x aims to maximize compatibility, so they allow 16-bit Windows programs to work very much like in Windows 3.x. 16-bit Windows programs can potentially hold the CPU forever, and a mechanism called Win16Mutex blocks all other user mode threads, including Windows built-in application threads (such as Windows Explorer, Task Manager, etc.) to execute. Unlike Windows 9x, Windows NT series doesn't aim for such high level of compatiblity. They are pure 32-bit platforms so all 16-bit programs run on virtual machines, which may not be very compatible but are secure.
There are also some differences between the Command Prompt's of NT and 9x. In Windows 9x, it is called MS-DOS Mode. By default, it is 25 lines in height. In Windows NT, it is called Command Prompt and by default it is 300 lines in height while the windows shows only 25 of them. There is a scroll bar in the right of the Command Prompt in Windows NT. In addition, when using Chinese under Chinese versions of Windows 9x, users can enter Chinese characters in 16-bit DOS programs through PDOS95, a program shipped with Windows 9x Chinese versions. Users can never enter Chinese characters in 16-bit DOS programs when using Windows NT. They can only enter Chinese characters in 32-bit Windows console programs by pressing shortcut keys that call out a input method. In order to let Windows NT Command Prompt support Chinese, the user can type a command "CHCP 936".
For Windows NT 4.0, Device Manager is not provided so every hardware component corresponds to a Control Panel item. A user should use these Control Panel items to install and manage hardware components.
Windows NT/2000 are different than Windows 9x in managing the computer. Administrators use a set of Administrative Tools to manage the computer. For Windows NT 4.0, these tools can be found in the Start Menu. For Windows 2000, these tools can be found in the Control Panel, and their interfaces are unified with MMC (Microsoft Management Console).
Windows NT/2000 support NTFS - Windows NT File System, a secure and efficient file system. With NTFS, files of users can be protected via a mechanism called DACL (Discretionary Access Control List). A user's file can be protected so that another user cannot access or change the file. This is quite useful when there are many users logging on to the same computer. Another mechanism called SACL (System Access Control List) allows auditing to see which user accessed which file in which manner. In Windows 2000, a new feature of NTFS - EFS (Encrypted File System; designed by Robert Gu) is introduced. It prevents particular files to be read without permission of the copy of Windows 2000 on the host computer. This is quite useful when there is someone that can physically touch the computer.