trying to understand fastcgi

[Matthew Ngaha]

I was trying to understand the fastcgi forwarding. The example shows:

server {
    server_name .website.com;
    listen 80;
    root /home/website/www;
    index index.html;
    location / {
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_param SCRIPT_FILENAME  $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_script_name;
        include fastcgi_params;
    }
}

in SCRIPT_FILENAME, does $document_root  ==  root /home/website/www; ?
and what value is usually stored in (SCRIPT_NAME)
$fastcgi_script_name? i don't understand what script SCRIPT_NAME
refers to. Am i suppose to set it or does it set itself?

It also says QUERY_STRING   $query_string; is needed for configuring
fastcgi. But this example clearly hasn't used it?

Is the idea to have fastcgi_param for only the variables i want to
include? or is this config simply defining them with a value by
default?