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.

You know that this was neither easy or cheap. So does everyone else! So, a merchant account shows that you've made a serious commitment to your business. That's good for customer confidence, which is good for business. So far so good...

Now there's issue of selling stuff to people online. Your order form leads them to feed their credit card info to a secure gateway, using software you bought or leased from (or through) your merchant account provider. Finally, transaction is approved or denied.

If approved, software generates a receipt and emails you and customer each a copy. At this point, customer is returned to a page you specified. In case of downloadable products, this is often page where they download your product. So, you've got entire process fully automated.

For a product or service with a fairly low price point and a potential for many thousands of sales, this seems ideal. You can quite literally make sales and earn income 24 hours a day. So, what's problem?

The form code on your order page is problem. If someone uses ViewSource function of their browser, they can see all your code. If they have even a tiny bit of initiative and skill, they can locate URL of your download page. After all, it's right there in your form code!

CGI provides two ways of fixing this problem. One involves using a script that makes it impossible to view source code. You can find a source for such a script by searching web. Expect to pay a lot for this technology.