LaTeX, HTML and XML

Hi.

I'm still slightly confused about the difference between XML and HTML.

I have attached a simple LaTeX screen dump which creates a .ps showing the
quadratic formula.

I would like to produce both HTML and XML to do the same thing for a web
page.

Can someone show me (literally give me the code if possible):

1. How to do this with HTML.
2. How to do this with XML.

I am familiar with the basics of HTML but don't how to incorporate nice math
content.

I know nothing about XML and don't quite understand where to begin with it.

Also,

3. Are HTML files "replaced" by XML files? ie. Are there web pages that use
*.xml extensions?
4. Or are the web pages still *.html but with XML employed within the *.html
document somehow?

Thanks in advance.

Russell.