HTML (hypertext markup language) is the formatting language (or code) the Web is based on. An HTML document looks quite different from the Web page the browser shows; the Web page you see on your screen is the browser's interpretation of an HTML document.

The images you see are called up by the HTML document, but they aren't part of it - they're separate documents. The browser reads the location of the images from the HTML code, then places them on the Web page alongside the text and other elements that are within the main document. Similarly, audio or movie files can be part of the set of documents that are called by an HTML file and assembled by the browser.

The browser cache is another important part of the picture: Because documents on the Web take time to download (as you may know all too well), browsers can store images temporarily in a cache on your hard drive to help you avoid having to download the same image more than once. This cache can and should be emptied at least once every four or five hours.

The ability to display and store Web pages is what makes a browser a browser, but, like with word- processing programs, there is more than one browser out there.

Why is there more than one Web browser? A little history might help here. Until a few years ago, the Internet dealt almost exclusively with text; then, in 1993, the National Center for Supercomputing Applications at the University of Illinois released Mosaic, the first graphical browser, and the World Wide Web began its explosive cycle of growth. The Web became so popular, one could argue, because people like to look at pictures, and the Mosaic browser allowed people to do just that.

The Mosaic browser made it possible to design documents containing images for display over the Internet, instead of simply putting a bunch of text on a server. The Web page was born.

The real action, however, didn't begin until 1994, with the release of Netscape Navigator 1.1.

Mosaic ship-jumper Marc Andreessen followed the successful model Mosaic had established when he released Netscape: Distribute the product for free on the Internet in order to establish a wide user-base. It worked like a charm. Netscape quickly eclipsed Mosaic and became the browser of choice, dominating the browser market to such an extent that - in the time-honored tradition of the Band-Aid and the Q-Tip - the name "Netscape" became synonymous with "browser."

That kind of total market saturation couldn't last long, though. Microsoft finally got hip to the potential of the Internet and set out - in its usual style - to outspend Netscape and build a better browser the following year. This game of catch-up has been very successful. The user base for Microsoft Internet Explorer continues to grow at a fast clip, largely due to the company's strategy of bundling Explorer with Windows system software.

And then there's America Online, the nation's largest online service and the company that now owns Netscape. Much of AOL's action takes place in its own online "environment," available only to subscribers. But it, too, offers its members a Web browser.