The namespace is usually just a string. It provides the mechanism to tell two namespaces apart. Strings such as "urn:www.microsoft.com," "http://www.microsoft.com," and "uuid:1234567890" are all valid names for a namespace.
The Internet Explorer 5 XML parser is a validating parser. When there is an XML schema that can be used to validate the structure and content of an element, the XML parser is asked to use the schema by using the following syntax:
<BOOK xmlns="x-schema:http://www.microsoft.com/BookInfo.xml">
<TITLE>A Suitable Boy</TITLE>
<PRICE currency="US Dollar">22.95</PRICE>
</BOOK>
This "x-schema" instructs the XML parser to treat the reference as a valid reference to an XML Schema; it fetches the schema from the URL and validates against it.
For more information on XML Schemas, see XML Schema Developer's Guide.