Unlike HTML,CGI is a...?

common gateway interface used as a protocol for HTML to interface with a scripting language such as PHP.

CGI stands for Common Gateway Interface.

CGI programs are programs that know how to interface with the web server software such as "Apache httpd". The CGI program can either read it's standard input or accept the program parameters from the "$QUERY_STRING" environment variable or both. In the case of Apache, other parameters are set within environment variables before the CGI program is executed.

Generally, CGI programs must output at least 1 http header before it can start sending the output of the CGI program. Most commonly, the output line is

Content-type: text/html;

followed by a blank line.

CGI scripts can be used to output web pages (html files) or any other type of output such as images, pdf files, XML files, SOAP responses, spreadsheets, word documents, .....

CGI programs/scripts can be written in C, PHP, Perl, shell scripts or any other language that can be executed on the web server environment.

.NET web pages served by ASP.NET are not generally considered CGI programs because the interface to the web server is different.

If a web page is written in perl or PHP, it generally follows the guidlines for being called a CGI program.

its server side...can be hidden from users