HTML Emails are usually email newsletters that have been signed up for and will be viewed as HTML by the Viewers. Unsolicited HTML email will be either consigned to the Spam/Junk folder so you need to make sure it is an email that people WANT to receive. Even those with Text only may switch quickly to HTML view for your newsletter.
There are many tutorials available on how to make HTML Emails in apps like Outlook, Outlook Express, Windows Mail, etc.. There's software available for actually making email newsletters. There are certain rules that must be followed for HTML emails versus an HTML web page.
Email Marketing Articles and Guides: http://www.mailchimp.com/articles/stupid-html-email-design-mistakes/
How to Create and Send HTML Email: http://www.addme.com/newsletters/issue293.htm
How to Create Great HTML Emails with CSS: http://www.webreference.com/programming/css_html/
30 Free HTML email templates: http://www.campaignmonitor.com/templates/
How do you typically create HTML email?: http://webdesign.about.com/b/2008/10/02/poll-how-do-you-typically-create-html-email.htm
How to Code HTML Email Newsletters: http://articles.sitepoint.com/article/code-html-email-newsletters#
Ron