In the MySpace sense, HTML is a basic set of codes that you can use to change how your text looks. You can also use it to insert graphics.
It's a bit complicated, but you might check out this site to get you started:
http://personalweb.about.com/cs/begi...basic_html.htm
EDIT: This one seems to be a bit simpler and more geared toward MySpace users.
http://help.nuclearcentury.com/showthread.php?t=4680