There were some questions asked about namespaces at the Annapolis CFUG talk on XHTML.

Namespaces are a way of indicating that an XML tagged piece of data is unique to something. That is, "<para>" could refer to a paragraph, a paramedic, or a paragraph. You can use a namespace prefix with this tag or wrap it with a namespace tag to associate it with that which gives it meaning, for instance a group of related tags also associated with the namespace. You use lines like:
<html: ... xmlns:er="http://somedomain.com/ER/ ..."> (paraphrased)
to say that the tags prefixed with "er:" are related and unique from say html tags or tags with another prefix. The "http://somedomain.com/ER/" does not have to even be a real URL, it is not looked up.

Namespaces can make it easier to write applications and scripts using XML by helping make things clearer.