The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces (APIs) available in the Microsoft Windows operating systems. The name Windows API collectively refers to several different platform implementations that are often referred to by their own names (for example, Win32 API); see the versions section. Almost all Windows programs interact with the Windows API. On the Windows NT line of operating systems, a small number (such as programs started early in the Windows startup process) use the Native API.

Developer support is available in the form of a software development kit, Microsoft Windows SDK, providing documentation and tools needed to build software based on the Windows API and associated Windows interfaces.

The Windows API (Win32) is focused mainly on the programming language C in that its exposed functions and data structures are described in that language in recent versions of its documentation. However, the API may be used by any programming language compiler or assembler able to handle the (well-defined) low-level data structures along with the prescribed calling conventions for calls and callbacks. Similarly, the internal implementation of the API's function has been developed in several languages, historically. Despite the fact that C is not an object-oriented programming language, the Windows API and Windows have both historically been described as object-oriented. There have also been many wrapper classes and extensions (from Microsoft and others) for object-oriented languages that make this object-oriented structure more explicit (Microsoft Foundation Class Library (MFC), Visual Component Library (VCL), GDI+, etc.). For instance, Windows 8 provides the Windows API and the WinRT API, which is implemented in C++ and is object-oriented by design.

Versions

Almost every new version of Microsoft Windows has introduced its own additions and changes to the Windows API. The name of the API, however, remained consistent between different Windows versions, and name changes were kept limited to major architectural and platform changes for Windows. Microsoft eventually changed the name of the then current Win32 API family into Windows API and made it into a catch-all term for both past and future API versions.

Win16 is the API for the first, 16-bit versions of Microsoft Windows. These were initially referred to as simply the Windows API, but were later renamed to Win16 in an effort to distinguish them from the newer, 32-bit version of the Windows API. The functions of Win16 API reside in mainly the core files of the OS: kernel.exe (or krnl286.exe or krnl386.exe), user.exe and gdi.exe. Despite the file extension of exe, these actually are dynamic-link libraries.
Win32 is the 32-bit application programming interface (API) for 32-bit versions of Windows (NT, 95, and later versions). The API consists of functions implemented, as with Win16, in system DLLs. The core DLLs of Win32 are kernel32.dll, user32.dll, and gdi32.dll. Win32 was introduced with Windows NT. The version of Win32 shipped with Windows 95 was initially referred to as Win32c, with c meaning compatibility. This term was later abandoned by Microsoft in favor of Win32.
Win32s is an extension for the Windows 3.1x family of Microsoft Windows that implemented a subset of the Win32 API for these systems. The “s” stands for “subset”.
Win64 is the variant of the API implemented on 64-bit platforms of the Windows architecture (as of 2021, x86-64 and AArch64). Both 32-bit and 64-bit versions of an application can be still compiled from one codebase, although some older APIs have been deprecated, and some of the APIs that were already deprecated in Win32 were removed. All memory pointers are 64-bit by default (the LLP64 model), so the source code must be checked for compatibility with 64-bit pointer arithmetic and rewritten as necessary.
WinCE is the implementation of the Windows API for the Windows CE operating system.