Browsers, such as Netscape, Internet Explorer, and others, interpret your documents for on-line presentation using special codes you insert in your documents. These codes, or tags, make up a markup language called Hypertext Markup Language (HTML).
You could use plain text files on the Web, but HTML gives your documents a more polished look and creates a more reliable document. (Keep in mind, too, that various browsers might handle plain text differently, and some don't understand line breaks at all, causing files without HTML codes to look like one long sentence.) HTML also enables you to make hypertext links to other files, allowing readers to skip from one file to another simply by clicking on a link that you included in your document.
In general, HTML defines the parts of your document. It's actually quite primitive compared to word processors and desktop publishing packages, but its simplicity helps to keep files small for quick transmission across networks, and it sets a standard that users can view on many kinds of computers using many different kinds of browsers.