The Windows API (Application Programming Interface) is a set of functions and tools provided by Microsoft that allows developers to create applications for the Windows operating system. It provides a way for software to interact with the hardware and system resources, enabling tasks like creating windows, handling user input, and managing files. Developers use the Windows API to build applications that can run on various versions of Windows. It includes libraries for graphics, networking, and user interface design, making it easier to develop software that is compatible with the Windows environment.