HTML is the markup language used to define the structure of hypertext documents on the World-Wide Web. In many ways it is similar to LaTeX or MML (the Maker Markup Language) although much less flexible (and thus much easier to learn). The best way to learn HTML is by example -- for instance, you can see the HTML code for this document by selecting "File/View Source..." from the Mosaic menu (or something similar on other browsers).
Once you have an idea of what HTML can do, start reading the variety of HTML tutorials, specifications, and style guides. Here's a list of the ones I know about (in order of increasing mastery of the language):