Source : Free On-Line Dictionary of Computing

CGI program
     
         (Often "CGI script") A program running on a
        {web server} to produce dynamic content, usually an {HTML}
        {web page}, in response to a user's request.
     
        The {Common Gateway Interface} specification defines the
        interface between the web server and such programs.  The
        program can access any data that a normal application program
        can, however the facilities available to CGI programs are
        usually limited for security.
     
        Although CGI programs can be compiled programs, they are more
        often written in a (semi) {interpreted language} such as
        {Perl}, or as {Unix} {shell scripts}, hence the name "CGI
        script".
     
        Here is a trivial CGI script written in Perl.  (It requires
        the "CGI" module available from {CPAN}).
     
         #!/usr/bin/perl
         use CGI qw(:standard);
     
         print header, start_html,
           h1("CGI Test"),
           "Your IP address is: ", remote_host(),
           end_html;
     
        When run it produces an {HTTP} header and then a simple HTML
        page containing the {IP address} or {hostname} of the machine
        that generated the initial request.  If run from a command
        prompt it outputs:
     
         Content-Type: text/html
     
         
         
         
CGI Test
Your IP address is:
         localhost
     
        The CGI program might be saved as the file "test.cgi" (or
        test.pl) in the appropriate directory on a web server,
        e.g. "/home/httpd/cgi-bin/".
     
        A user could then type the appropriate {URL},
        e.g. http://www.acme.com/cgi-bin/test.cgi, into their {web
        browser} to get the program to run and a custom page produced
        for them.
     
        Early web servers required all CGI programs to be installed in
        one directory called cgi-bin but it is much better to keep
        them with the HTML files to which they relate unless they are
        truly global to the site.  All modern web servers make this
        easy to do.
     
        (2000-06-23)