File Explorer

File Explorer is the application responsible for the Windows user interface and the default Windows file manager. The first build to include it was Windows 95 build 58s, under the name of Cabinet for the shell portion and File Cabinet for the file manager part. Build 189 is the first build leaked with Cabinet renamed to Windows Explorer. During the development of Windows 95, several Shell Technology Previews were released for Windows NT 3.51, that brought the new user interface to the NT series. Internet Explorer 4 updated Windows Explorer with a new, web-integrated experience. Windows Vista saw major changes to Explorer, simplifying the UI somewhat, as well as removing features Microsoft thought were under-utilized. Windows 7 added Libraries to the Explorer experience. In Windows 8, Windows Explorer was renamed to File Explorer.

File Cabinet
Explorer was first introduced in Windows 95, but it did not start out looking like its final design. In early builds, Explorer was named Cabinet, the Taskbar acted as a folder, and the Start Menu was not present; It was instead divided into three separate menus, each providing a portion of the final functionality. The first menu (denoted by the Windows logo) acted as a system menu where you could access applications using the 'Run' dialog, manipulate windows and shutdown Windows. The second menu (denoted by a magnifying glass) provided Windows Search-like functionality. The last menu was the 'Help' menu where you could access the Windows documentation and get basic system information.

Shell Technology Preview/NewShell
NewShell is a pack that adds the new NT4 (alpha) shell to NT 3.51. Only two builds of NewShell, build 1054 and 1057 has been leaked as of 2021. NewShell's UI is similar to that of Windows NT 4.0 build 1130.1. It causes the kernel version to jump to (NT) 4.00. A final (RTM) version of NewShell was never released.

Longhorn
Windows Longhorn saw many changes to Explorer. The .NET Framework included a  namespace. A sidebar was integrated into Explorer, and in certain builds could even be integrated with the taskbar, as well as the Avalon (Windows Presentation Foundation) technology which changed the standard view and provided capabilities beyond what Windows Forms could do. Libraries were also intended to be included in Windows Longhorn but was ultimately scrapped when development was reset until Windows 7.