Give people a few hints, and they can figure out the rest. They can look at this page, see some large type followed by blocks of small type and know that they are looking at the start of a magazine article. They can look at a list of groceries and see shopping instructions. They can look at some rows of numbers and understand the state of their bank account.

Computers, of course, are not that smart; they need to be told exactly what things are, how they are related and how to deal with them. Extensible Markup Language (XML for short) is a new language designed to do just that, to make information self-describing. This simple-sounding change in how computers communicate has the potential to extend the Internet beyond information delivery to many other kinds of human activity. Indeed, since XML was completed in early 1998 by the World Wide Web Consortium (usually called the W3C), the standard has spread like wildfire through science and into industries ranging from manufacturing to medicine.

The enthusiastic response is fueled by a hope that XML will solve some of the Web's biggest problems. These are widely known: the Internet is a speed-of-light network that often moves at a crawl; and although nearly every kind of information is available on-line, it can be maddeningly difficult to find the one piece you need.

Both problems arise in large part from the nature of the Web's main language, HTML (shorthand for Hypertext Markup Language). Although HTML is the most successful electronic-publishing language ever invented, it is superficial: in essence, it describes how a Web browser should arrange text, images and push-buttons on a page. HTML's concern with appearances makes it relatively easy to learn, but it also has its costs.

One is the difficulty in creating a Web site that functions as more than just a fancy fax machine that sends documents to anyone who asks. People and companies want Web sites that take orders from customers, transmit medical records, even run factories and scientific instruments from half a world away. HTML was never designed for such tasks.

So although your doctor may be able to pull up your drug reaction history on his Web browser, he cannot then e-mail it to a specialist and expect her to be able to paste the records directly into her hospital's database. Her computer would not know what to make of the information, which to its eyes would be no more intelligible than < H1 >blah blah < /H1 > < BOLD >blah blah blah < /BOLD >. As programming legend Brian Kernighan once noted, the problem with "What You See Is What You Get" is that what you see is all you've got.

Those angle-bracketed labels in the example just above are called tags. HTML has no tag for a drug reaction, which highlights another of its limitations: it is inflexible. Adding a new tag involves a bureaucratic process that can take so long that few attempt it. And yet every application, not just the interchange of medical records, needs its own tags.

Thus the slow pace of today's on-line bookstores, mail-order catalogues and other interactive Web sites. Change the quantity or shipping method of your order, and to see the handful of digits that have changed in the total, you must ask a distant, overburdened server to send you an entirely new page, graphics and all. Meanwhile your own high-powered machine sits waiting idly, because it has only been told about < H1 >s and < BOLD >s, not about prices and shipping options.

Thus also the dissatisfying quality of Web searches. Because there is no way to mark something as a price, it is effectively impossible to use price information in your searches.

Something Old, Something New

The solution, in theory, is very simple: use tags that say what the information is, not what it looks like. For example, label the parts of an order for a shirt not as boldface, paragraph, row and column--what HTML offers--but as price, size, quantity and color. A program can then recognize this document as a customer order and do whatever it needs to do: display it one way or display it a different way or put it through a bookkeeping system or make a new shirt show up on your doorstep tomorrow.