Unfortunately, not many people know the difference. Most of us are probably sending and receiving formatted HTML email or Rich Text (RTF) without knowing it and HTML emails are ideal if you are sure that your recipients can view them.

Not everybody, who gets HTML email likes it and not everyone, uses an email client that can display HTML HTTP is described and standardized in rfc 1945...

Different email clients or browsers display HTML message differently. Therefore, not everyone can open and view your content just as you intended it to be. If you only design around Outlook and Internet Explorer you’re ignoring potential viewers with various other clients.

Most of the time standard HTML formats work just fine but will be quite a problem using Flash, iFrames and scripts of any kind–depending on user's clients security settings. Many ISP free mail like “Hotmail” don’t allow Flash, iFrames, scripts, etc., to get though to the recipient and not using those features should be considered seriously....

The differences between HTML and XHTML are small. As the rules of XHTML are based on those of XML, the same exceptionally strict rules apply as with XML. The major differences are outlined below.

The most important difference between the two markup languages is that HyperText Markup Language, or HTML, is an application of SGML (Standard Generalized Markup Language)1 and allows an author to omit certain tags and use attribute minimization. The Extensible HyperText Markup Language, or XHTML, is an application of XML (Extensible Markup Language). It doesn’t permit the omission of any tags or the use of attribute minimization. XHTML documents must start with an XHTML DocType declaration....

Hypertext Markup Language. A standardized language of computer code, imbedded in “source” documents behind all Web documents, containing the textual content, images, links to other documents (and possibly other applications such as sound or motion), and formatting instructions for display on the screen. When you view a Web page, you are looking at the product of this code working behind the scenes in conjunction with your browser. Browsers are programmed to interpret HTML for display.

HTML often imbeds within it other programming languages and applications such as SGML, XML, Javascript, CGI-script and more. It is possible to deliver or access and execute virtually any program via the WWW. ...