Windows Services for UNIX (SFU) or Subsystem for UNIX-based Applications (SUA) is a software package produced by Microsoft which provides a Unix subsystem and other parts of a full Unix environment on Windows NT and some of its immediate successor operating-systems. It was an extension and replacement of the minimal Microsoft POSIX subsystem from Windows NT.
SFU 1.0 and 2.0 (NT 4.0 and Windows 2000) used the MKS Toolkit; starting with SFU 3.0 (Windows XP; 1999), SFU used the Interix subsystem, which was acquired by Microsoft in 1999 from US-based Softway Systems as part of an asset acquisition. These are available free from Microsoft as a download. Windows Server 2003 R2 included SFU 3.5 (on Disk 2), and renamed it to Subsystem for UNIX-based Applications (SUA). In Windows Server 2008 and high-end versions of both Windows Vista and Windows 7 (Enterprise and Ultimate), a minimal SUA is included, but most of the utilities must again be downloaded from Microsoft's web site.
Like the Microsoft POSIX subsystem in Windows NT that it replaces, Interix is not an emulation of a Unix kernel, but rather an implementation of a user-mode subsystem running on the Windows NT kernel.
Windows Services for Unix and Subsystem for Unix-based Applications provide header files and libraries that make it easier to recompile or port Unix applications for use on Windows; they do not make Unix binaries compatible with Windows binaries. It is best thought of as a distinct Unix-like platform.