Except for the fact that both are streams of plain text, HTML and RTF have nothing to do with each other. If you open an example of each in a plain text editor, then you will easily see the differences. Rich text is highly coded and is generally indecipherable by human beings. It contains no hyperlinks. HTML is plain text with a few tags to handle formatting and hyperlinks. It is generally readable by most human beings.
Mail allows you to send and receive HTML mail, but it does not include the tools to create it. This will change with MacOS X 10.5. For now, you must create your HTML message using an external HTML editor and and then paste it into Mail.