A Microsoft set of standard application programming interfaces (APIs) for developing extensions to Internet Information Services (IIS). Internet Server API (ISAPI) provides developers with a powerful way to extend the functionality of IIS to provide better performance than Active Server Pages (ASP) or the Common Gateway Interface (CGI) and to provide low-level access to all Microsoft Win32 API functions. Of course, since ISAPI dynamic-link libraries (DLLs) are generally written in a high-level programming language such as C or C++, they are usually more difficult to develop than ASP-based or CGI-based solutions and might not scale as well.
There are two kinds of ISAPI DLLs, which have different uses on IIS:
You can create ISAPI extensions easily using the ISAPI Extension Wizard in Microsoft Visual C++.