What is "xml-namespaces"

XML namespaces are described in standard W3C XML Names:

XML namespaces provide a simple method for the qualification of element names for Qualifying element and attribute and attributes used in XML documents by associating them with namespaces identified by references in URI format.

Although many namespaces assume the format of Urls, they do not necessarily refer to an Internet location.

More information (SO-EN): What are XML Namespaces for?