A good example of a this phenomenon is a simple CGI routine called FormMail. This was written a number of years ago by a fellow named Matt Wright to allow data to be entered in a form, then emailed to a recipient.

I first looked at FormMail because I wanted to cut down on spam. You see, my web site had my email address embedded on every single page. I thought this was a good idea to allow people to send me an email message when they wanted to contact me. In fact, all of web design books indicate that all good web sites include an email link of this kind.

I soon discovered, much to my horror, that spammers use special programs called Spam Harvesters to scan websites for email addresses. They add these addresses to their mailing lists and resell them over and over. The result is a large increase in amount of spam that I received.

After much research, I came to conclusion that best defense against spam robots was to simply stop including my email address on my web sites. This left question of how to allow users to contact me when they had questions or comments.

The answer is simple - use a form. The advantage is that email address is hidden within CGI routine or a text file and it is simply not possible for a spam harvester to pick it up. As long as email address is coded into CGI routine or in a database you are relatively secure.

However, many people use FormMail in a different way. Let's say you want to allow your visitors to "tell a friend" about your site. So you include a form which allows visitors to enter their message and a target email address. If you are not very careful you could find that you have set yourself up as a spam relay.

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 data.

Some of more common uses of CGI include:

- Guestbooks - The CGI routine is responsible for accepting data, ensuring it is valid, sending an email acknowledgement back to writer, perhaps sending an email to webmaster, and creating guestbook entry itself.

- Email Forms - A simple CGI forms routine just formats data into an email and sends it back to webmaster. More complicated routines can maintain a database, send an acknowledgement and validate data.

- Mailing List Maintenance - These routines allow visitors to subscribe and unsubscribe from a mailing list. In this case, CGI routine maintains a database of email addresses, and better ones send acknowledgements back to visitor and webmaster.

A CGI routine can be anything which understands CGI standard. A popular CGI language is called PERL, which is simple to understand and use (well, compared to other languages). PERL is a scripting language, which means each time a PERL routine is executed web server must examine PERL commands to determine what to do. In contrast, a compiled language such as C++ or Visual Basic can be directly executed, which is faster and more efficient.

Okay, in a nutshell (and greatly simplified), here's how it works:

1) You (the webmaster) specify a form tag which includes name of CGI routine.