This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I don't think so. I am not sure what you mean by "identify an XML" and I don't know what you mean by "globally", but at any rate a namespace is used to signal to an application program that an XML document has certain semantics and should be processed in a certain way.