Click on the link below. If it helps any, CGI means Common Gateway Interface. Typically, you use standard HTML tags to get data from a person, then pass that data to a CGI routine. The CGI routine then performs some action with the data. Email Forms - A simple CGI forms routine just formats the data into an email and sends it back to the webmaster.
More complicated routines can maintain a database, send an acknowledgement and validate data.
These routines allow visitors to subscribe and unsubscribe from a mailing list. In this case, the CGI routine maintains a database of email addresses, and the better ones send acknowledgements back to the visitor and webmaster. A CGI routine can be anything which understands the CGI standard. PERL is a scripting language, which means each time a PERL routine is executed the web server must examine the PERL commands to determine what to do. You create HTML tags which retrieves data from your visitors.