There are two major types of plug-ins for the Microsoft WEB server products
The ISAPI applications are intended to do drive a WEB site or act as engine for which the actual server side code is
written (e.g. ASP is an ISAPI application).
The ISAPI filters on the other hand are plug-ins that interact with the work the WEB server does and perform additional
actions, alter certain data or replace some of the server's functionality. The ISAPI filters are usually independent
of the nature of the WEB content and the WEB applications running on the server. Still, very often they provide features
these applications expect. For example an authenitaction filter makes possible to use custom user lists over which to
authenticate the visitors instead of creating a system account for each of them. This can be "a must" for certain
application, yet it is not part of it and there is no matter in what kind of language it is written.
ISAPI
or Internet Server Application Program Interface is a set of Windows program calls that
let you write a Web server application that will run faster than a common gateway interface application.