* Kindly shift this to appropriate sub-topic if this is not the correct one.

I am having trouble understand what an interface between web server and web applications mean. Maybe this is because of incomplete knowledge.

What does WSGI, CGI etc. do?
As far as I understand, a machine gets a request, it passes on the request to the framework (or the web server) that is running (like Django, Drupal etc.) which then looks up something like index.php or index.py, does some processing on it and returns a HTML page.

Where in the above process is WSGI, CGI involved?