The Windows application programming interface (API) is the user-mode system programming
interface to the Windows operating system family. Prior to the introduction of 64-bit versions of
Windows, the programming interface to the 32-bit versions of the Windows operating systems was
called the Win32 API to distinguish it from the original 16-bit Windows API, which was the program-
ming interface to the original 16-bit versions of Windows. In this book, the term Windows API refers
to both the 32-bit and 64-bit programming interfaces to Windows.