All application programs (the kind of programs developed with such languages as Visual Basic) must be developed to work with a specific operating system. Application Programs interact with operating systems by using operating system functions that applications developers can exploit. These functions are usually referred to as application program interface (API) functions. API functions allow programmers to add such functionality to their programs as opening and saving files, checking system hardware, using the computer’s internal clock, and, in the Microsoft Windows family of operating systems, creating, modifying, and communicating with Windows.
The Windows API is enormously complex. It has over 800 functions, some of which can perform a wide variety of tasks. The value of Visual Basic is that it greatly simplifies Windows programming. Visual Basic encapsulates the Windows API into objects that Visual Basic programmers can manipulate. For example, when you use the C programming language, creating a program that greets the user with the message “Welcome to Visual Basic” can take several lines of instructions. In Visual Basic, however, the same program requires only a single line.