A URN is a name and doesn't represent a location.

So it is great for document namespaces.

It has a special syntax and starts with the scheme 'urn'.

The parts of the name are colon separated.

Following the scheme is an NID (Namespace ID)--which are restricted. You can't make up your own NID!!!

For example, the namespace for my slide content is:

urn:publicid:IDN+mathdoc.org:schema:slides:2004:1.0:us