<HR ...> draws a horizontal line (a "horizontal rule") across the page. That's it. For such a simple concept, horizontal rules are suprisingly popular. Let's start with the basics.
<HR ...> has no end tag and requires no attributes:
creates this rule:
<HR ...> is usually indicates a division in the page. Stuff before the rule is in a different "section" from the stuff after. For that reason many designers consider
<HR ...> a logical tag, not a layout tag.