ISAPI filters are similar to ISAPI extensions in that they respond when IIS receives an HTTP request. They are different from extensions in that they are driven by IIS events rather than by a client request. You can associate an ISAPI filter with a particular IIS event; the filter is then notified every time its associated event occurs.
Filters are also different from extensions in that the data structure sent from IIS to filters is determined by the type of event any particular filter is associated with. IIS always presents an extension with request data through an ECB. The following diagram illustrates interaction between IIS and an ISAPI filter.
ISAPI filters are very powerful and can be used to facilitate applications that do a number of different tasks, including
For further information on how filters process notifications, see Filter Event Processing.