The Microsoft Windows API provides a set of functions that are generally supported on both 32 and 64-bit Windows.
Originally called the Win32 API, the name Windows API its roots in 16-bit Windows and its current support of 32 and 64-bit Windows.
Tk and Tcl use the Win32 API to access OS functions and draw on the screen.
[elaborate on how it can be used for many other purposes and how Tcl uses it]