Hi John,
FormBox is for hosting a Visual WebGui form within an ASP.NET (aspx) page. HtmlBox is for hosting of Url or Html within a Visual WebGui application. The HtmlBox is an IFrame based control where you can place (almost) any Url, but as with the FormBox, the page hosted within the HtmlBox is a seperate application, even from a different domain, so it's disconnected from your Visual WebGui application unless you add your own communication channels. See more here.
Palli