If you wish to get more advanced information about HTML, you might want to use some of the following reference sites. These are references and aren't too helpful at explaining things to new users, though. They may be more useful to you later on in the course.
I don't really recommend you use these sites to learn more HTML because it is very easy to pick up "bad habits"; a lot of the HTML tags that exist really shouldn't be used. Obviously, as the course progresses I'll cover more of the language and will also show the "correct" ways to do some of the things you might want to do using the more dubious HTML tags (e.g. FONT - avoid this tag at all costs :)
There are hundreds of HTML tags and you really don't need to use more than a handful of them in producing a Web site. The course Web site only uses 18 different tags: !DOCTYPE, HTML, HEAD, TITLE, BODY, H1 .. H4, P, IMG, UL, LI, A, LINK, STYLE, DIV, and SPAN. All these tags (and probably a few more) will be covered during the course, as well as the CSS language that you use to add graphical style to the HTML structure.
A useful and cleanly designed reference site. Contains a full reference to HTML 4.0 (the current standard which we're using) and CSS. If you click the "offline versions" link, you can download the references either in HTML format, or if you use Windows, in convenient Windows Help format.
Index Dot HTML
Good reference covering all HTML tags (and CSS too) along with information about browser support. Slightly ugly, but contains a lot of useful information.