Hear me now and believe me later. If you thought a Web site consisted of HTML pages organized as a directory, go back to the 20th century. A successful Web site today consists primarily of XML code and a database.

HTML (www.w3.org/markup) still has its uses, and it is the root of XML (www.xml.org). The idea of creating tags, the structure of tags and many of the tags themselves are common to both schemas. But HTML only allows you to create static pages for viewing on a Web browser. If you want to enter the 21st century Web, you need XML.

XML is a system that lets you make up your own tags, rather than depending only on those defined by the World Wide Web Consortium. It was originally created as a way to break the logjam of proposals and counterproposals for the growing number of new tags that were required to define new content.

But XML has become much more than that.

XML lets you define any type of form. It lets you view and complete that form on your Web site. And it lets you translate the results back into any format your computer (or your business partner’s computer) may have. This makes it a great replacement for Electronic Data Interchange (www.wedi.org), which is a system for electronic invoicing that dates back to the 1980s.

With XML, standard business forms can be passed around the Internet and still translate back to ancient mainframes. Most industries today have XML standards organizations–like adXML for the advertising industry (www.adxml.org)–that work to standardize forms (and tags). You should find and join your industry’s group.

XML allows you to separate content–what you have–from presentation, which is how it looks. This lets you write separate presentation systems, or templates, for interactive TV, broadband, PDAs and wireless devices. The device that accesses your site will be directed to the correct template. XML also lets you change the look and feel of your Web site quickly–you don’t have to edit every page. Change the template, and the entire site’s appearance changes.

Most important, however, XML templates are the perfect interface for a database. All great sites today are based on databases because most computer data consists of databases.

Your company probably has a ton of different databases, such as customer, prospect, product, employment and financial information databases. Once your content becomes a database, you can make the whole thing searchable with a little box, just like on the Web search site Google. (If your content isn’t yet a database, Google can index it for you. For more information, visit www.google.com/services/index.html#custom_site.)

With XML, all these databases can be presented on your Web site. You’ll need buttons, tags and lists for major sections of your site, but in order to create a site of unlimited depth you have to build it as a database. There’s just no other way.

I’ve been covering the Web on a full-time basis since late 1994. The best example I can offer of a site that combined XML and databases to build commerce went online very early in my career–iQVC (www.qvc .com), the online unit of the popular home shopping channel.

The basic site design of iQVC has changed very little from its launch in 1995. The goal was to be different from its parent network QVC. On the Web site, the company could offer tens of thousands of products, instead of the few dozen it presented on television.

First, iQVC built a huge inventory database. Since QVC already had hundreds of thousands of customers, it quickly connected this database to the Web site. Because QVC’s sales pitch was based on personal contact between the on-air host and the customer, the company created a discussion forum, which was also organized as a database. And from the start iQVC interfaced those databases to the rest of the Web.

It can be difficult to create a system in which a product database interacts with a search engine. You will certainly want to write some static pages that search engines can find. But iQVC did it the right way from the very beginning, separating content from presentation and building its business on databases. As a result, iQVC is one of the most well-respected general merchants on the Web and one of the most profitable as well.

So learn and use XML. If you have to invest a pot of money, invest it in a database and a database design. Separate your data from how it looks, give yourself the opportunity to store (and find) a whole lot of stuff, and your Web site will do well.