Ugly URLs are URLs that expose details of a given implementation and hide (amid the clutter) the details that the users care about. The concept has been around for a while, but like air pollution recognizing the problem is just the first step.
Examples:
http://www.mysite.com/nuke/modules.php?name=Forums&file=posting&mode=quote&p=14488&sid=13d0675890f2f44b5278c61552f157a9Contrast with PrettyURLs.