An HTML module is a little editor that you can plop on to your page in the Page Editor, just like any other module. It holds HTML markup. With an HTML module, you can construct a web page with raw HTML markup just as the pros do.
If you're conversant with HTML, the idea of having a fine-grained control over your HTML is likely to pump you up. And yet, I don't recommend using HTML modules.
You read that right: I don't recommend using HTML modules. Why? Because Office Live Small Business stores the markup in HTML modules separate from the page. When a person requests your page from his/her browser, Office Live Small Business fetches the separately-stored markup and embeds it into the rest of the page dynamically. The problem is that it does the embedding using something called frames, which happen to be little windows on a web page that display content that doesn't actually reside on that page.
Why is this problematic? Because search engines can't index the content that doesn...