How application Programs work
Application programs are written in a text based computer language as mentioned in the section about hardware and software. Once written, they are complied into a binary language the computer understands. The application programs use function calls (as described in the section about operating systems) to interface to the various computer peripherals such as your keyboard, mouse, screen , printer and other devices. Most of these function calls are provided by the operating system so the application programs are usually complied for a specific operating system such as Microsoft Window 2000, Microsoft Window XP, or Microsoft Window 7 and Microsoft Window 8.
An application program is a computer program. It is written by human programmers who make mistakes. Therefore there can be errors in the code even though there may be some testing before the product is released. Application programs very widely in the quality of the code. Errors in code are referred to as ‘bugs’. Bugs can cause unpredictable result including system crashes, inability to perform expected functions, or providing an exploit for an attacker who wants to again control of your system.