API is some predefined interfaces (such as functions and HTTP interfaces), or refers to the conventions for connecting different components of a software system. Used to provide a set of routines that applications and developers can access based on a piece of software or hardware without accessing the source code or knowing the details of the internal working mechanism.
Classification:
The API function is contained in the dynamic link library file under the Windows system directory. Windows API is a set of predefined Windows functions, which are used to control the appearance and behavior of various Windows components.
Every action of the user will cause one or several functions to run to tell Windows what happened. This is very similar to the natural code of Windows to some extent. Other languages just provide an automatic and easy way to access the API.
When clicking a button on the form, Windows will send a message to the form, and VB will get this call and generate a specific event after analysis.
Refer to the above? Baidu encyclopedia -—API