ISAPI is split into two major components. ISAPI Applications are conceptually an in-process, free-threaded version of CGI while ISAPI Filters conceptually sit between the HTTP server and the HTTP socket. While ISAPI applications can be used to enrich the HTML pages and to provide dynamic data (like CGI), ISAPI filters can be used to add new authentication schemes, support new encryption or compression methods, change the content based on the client or other conditions, or provide enhanced logging capabilities.

Writing Internet Server Applications