Windows applications use the Win32 application programming interface (API), which is implemented by the Win32 subsystem. Programs written for MS-DOS, OS/2, Windows 3.x, and POSIX run in their own environmental subsystems, all of which interact extensively with the Win32 subsystem to implement their functionality.
The Interix subsystem is an independent subsystem that implements POSIX application programming interfaces. Even with this independence, you can still run Win32 programs, such as Notepad (notepad.exe) and Calculator (calc.exe) from the Interix shell prompt.