API (Application Programming Interface) is a set of predefined functions, or an agreement for connecting different components of a software system. It is used to provide a set of routines that applications and developers can access based on certain software or hardware without having to access the source code or understand the details of the internal working mechanism.

The operating system is the interface between the user and the computer hardware system. With the help of the operating system, the user can quickly, effectively, safely and reliably manipulate various resources in the computer system to process their own programs. API is the calling interface that the operating system gives to the application. The application makes the operating system execute the command (action) of the application by calling the operating system's API. In Windows, the system API is provided in the form of function calls.

The application program interface, also known as the application programming interface, is a set of definitions, programs and protocols that enable mutual communication between computer software through the API interface. One of the main functions of API is to provide a common set of functions. API is also a kind of middleware that provides data sharing for various platforms. In the practice of programming, the design of the programming interface must first make the responsibilities of the software system reasonably divided. Good interface design can reduce the interdependence of various parts of the system, improve the cohesion of the components, and reduce the degree of coupling between the components, thereby improving the maintainability and scalability of the system.