Question

What is ISAPI or ISAPI extension or filters? The more I read the more I am confused.

Answer 1

See e.g. here: http://searchwindowsserver.techtarget.com/definition/ISAPI

ISAPI (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
(CGI) application. A disadvantage of a
CGI application (or “executable file,”
as it is sometimes called) is that
each time it is run, it runs as a
separate process with its own address
space, resulting in extra instructions
that have to be performed, especially
if many instances of it are running on
behalf of users. Using ISAPI, you
create a dynamic link library (DLL)
application file that can run as part
of the Hypertext Transport Protocol
(HTTP) application’s process and
address space. The DLL files are
loaded into the computer when HTTP is
started and remain there as long as
they are needed; they don’t have to be
located and read into storage as
frequently as a CGI application.

A special kind of ISAPI DLL is called
an ISAPI filter, which can be
designated to receive control for
every HTTP request. You can create an
ISAPI filter for encryption or
decryption, for logging, for request
screening, or for other purposes.

Or see another definition with a graphical explanation here:

ISAPI Definition from PC Magazine