An ISAPI filter catch HTTP request and can make some pre-processing task
like authentication, compress, .... before IIS processing.
An ISAPI extension allow to call a specific task for an URL extension like
calling PHP EXE for .PHP, ASP DLL for .ASP, ...
Try the newsgroup "microsoft.public.platformsdk.internet.server.isapi-dev"
for more.
Lionel.
