XML ensures portability of data similar to Java, which ensures portability of code.

XML removes two constraints which were holding back Web developments:
1. dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;
2. the complexity of full SGML, whose syntax allows many powerful but hard-to-program options.
XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.

You can't.  XML itself is not a programming language, so XML files don't 'run' or 'execute'. XML is a markup specification language and XML files are just data: they sit there until you run a program which displays them (like a browser) or does some work with them (like a converter which writes the data in another format, or a database which reads the data), or modifies them (like an editor).

  No. XML itself does not replace HTML. Instead, it provides an alternative which allows you to define your own set of markup elements. HTML is expected to remain in common use for some time to come, and the current version of HTML is in XML syntax. 

 XML namespaces are designed to provide universally unique names for elements and attributes

You can use  XML import and export modules to connect XML applications with databases. In some trivial cases there will be a 1:1 match between feildnames in the database table and element type names in the XML Schema or DTD, but in most cases some programming will be required to establish the desired match. This can usually be stored as a procedure so that subsequent uses are simply commands or calls with the relevant parameters.

XML is a project of the World Wide Web Consortium (W3C), and the development of the specification is supervised by an XML Working Group. XML is a public format: it is not a proprietary development of any company, although the membership of the WG and the SIG represented companies as well as research and academic institutions