Win32 refers to the collection of Windows application programming interfaces (APIs) for developing 32-bit programs. Now that Windows supports 64-bit processors and applications, the API set is now known as Windows API (or WinAPI).
Microsoft provides a software development kit, the Windows SDK, to developers to use to build software using the Windows API. The API provides software developers with a common set of tools and commands for their software to interact with the Windows operating system and the computer's hardware. For example, it includes a common framework for application windows, dialog boxes, scroll bars, and other user interface elements. It also provides ways for the program to interact with files, connect to hardware devices, and interact with other computers over a network.
Win32 / Windows API is not the only API that software developers can use to write programs for Windows. Other APIs Microsoft has introduced for Windows include .NET, Universal Windows Platform (UWP), and WinRT. However, the evolution of the Windows API through Win16, Win32, and the modern WinAPI affords a high level of backward compatibility. Software written for old versions of Windows can still run on newer versions, with some extra compatibility work done by the operating system.