Many content management systems use url's containing ampersands to send parameters along with the url. For example the url of this page is "http://www.factsandpeople.com/index.php?option=com_content&view=article&id=113". But when you use such an url in the href attribute of an html "A" tag then do not forget to use "&" instead of "&" to make your html valid xml.

It is not very important because if you don't most browsers will still display the link as expected.