Read this chapter for a brief introduction about the Internet Server Application Programming Interface (ISAPI).
13.1. What is Internet Server API
Internet Server Application Programming Interface (ISAPI) is a multi-tier application programming interface for Microsoft Internet Information Services (IIS) web servers, and other compatible third-party web servers.
Two application types exist for ISAPI applications:
ISAPI applications are implemented by compiling Extensions or Filters into Dynamic Link Library (DLL) files. These DLLs must then be registered with the web server before they are available for use.