HTML ``expands'' to ``Hypertext Markup Language'', and is the most common format of data served by Web servers. When displayed with a graphical browser such as Netscape, you will see text information, pictures, and icons. When displayed with a text browser (or read by computer to a blind person), you will of course lose the pictures and icons, but the text will still show up -- at least if the Web page is properly written.
It is based on ``SGML '', which explains some of the peculiarities of the format -- such as the lack of font controls, and layout which tends to infuriate those who are more used to conventional publishing. The earlier versions of HTML were particularly bad for this, but the latest versions include such things as tables, some font control, etc...
Although there is a standard for what HTML is (and what it can do), the two main competing Web browsers -- Netscape's Communicator, and Microsoft's Internet Explorer each have their own enhancements to HTML. So there may be Web pages that are only properly visible with one or other of these browsers. Not many however, as most sensible Web authors tend to stick to the standard.
In addition to elements to specify the formatting of a document, HTML also has some features which allow forms to be created. This allows someone to write a Web document which with the help of readers allows him or her to collect some data.