Win16 was the first, 16-bit version of these APIss.
Win32 is the 32-bit API for modern versions of Microsoft Windows. The API consists of C functions implemented in dynamically linked libraries (DLLs), mainly in core DLLs: kernel32.dll, user32.dll and gdi32.dll. Although Microsoft's implementation of the Windows API is copyright, it is generally accepted that other vendors can emulate Windows by providing an identical API, without breaching copyright.
The Windows API was originally designed to be object oriented, but computers at the time were limited in power and many new APIs and enhancements have been introduced, leading to the loss of some object-oriented aspects. In some ways it is a low-level interface, so many programmers prefer to use the MFC (Microsoft Foundation Classes) and other class libraries such as Borland's OWL (Object Windows Library) for a more high-level approach. The recently introduced .NET libraries, commonly described as a replacement for calling the Windows API, are a wrapper around the Win32 API to provide object-oriented functionality and operating system abstraction.
Among other things, the Windows API contains the graphical widget toolkit used in different Windows versions.
The WINE project is an attempt to provide this API set for UNIX-like platforms.
Among common API functions are: