is a Microsoft-created API that stands for Internet Server API. In concept, ISAPI is similar to Apache’s Web Server API and Netscape Web Server’s NSAPI or WAI interface.
Essentially, these APIs allow programmers to write hooks into the various stages of Web server processing. These hooks are typically program modules that do things
like authenticate a user, but they can also be programmed to be a full-blown application. In the case of ISAPI Perl, Perl itself is embedded into the Web server using the ISAPI interface.
The difference between ISAPI and CGI is that ISAPI programs remain loaded in the Web server memory after an ISAPI program has first been loaded in memory. Thus, ISAPI programs tend to be much faster than CGI programs that are started and cleaned up
for each and every Web application request from the user’s Web browser.