I am an absolute newby to XML. Would you please explain what a namespace is, and how it is used?

Code:
<?xml version="1.0"?>
<HTML xmlns:xsl="http://www.w3.org/TR/WD-xsl">
  <BODY STYLE="font-family:Arial, helvetica, sans-serif; font-size:12pt;
        background-color:#EEEEEE">
    <xsl:for-each select="CATALOG/CD">
      <DIV STYLE="background-color:teal; color:white; padding:4px">
        <SPAN STYLE="font-weight:bold; color:white"><xsl:value-of select="TITLE"/></SPAN>
        - <xsl:value-of select="ARTISTe"/>
      </DIV>
    </xsl:for-each>
  </BODY>
</HTML>
In the above XSL what is the use of http://www.w3.org/TR/WD-xsl ? Do I have to be connected to the internet to view the results? What does the above XSL mean?