What Application is

Used interchangeably with program and software, this is a general term for a program that performs specific tasks, such as word processing, database management, e-mail sending or retrieval, or Web browsing. Unlike system software, which maintains and organizes the computer system (such as the operating system), an app is an end-user program.

The meaning of ISAPI

Internet Server Application Program Interface. An application programming interface created by Process Software and Microsoft. It is tailored to Internet servers. ISAPI uses dynamic link libraries (DLLs) to make processes faster than under regular APIs.