Namespace allows you to import other wsdl files, and not have to worry about tag names clashing. If you didn't use a name space than you would have to check, somehow, that any other wsdl which was used didn't have tags with the same name(s).
Target is where the web service lives.
Target namespace is a way of ensuring that your taget tag doesn't clash with anything else in your wsdl.
Target namespace (scroll to the bottom of this page)