What can you do with XML

The short answer is "anything your boss wants."

A somewhat longer answer:

Locally, XML can be used to store configuration files, attach meta-data to documents (information about the document) in a well-defined, stable yet extensible format that can be processed using widely available XML tools.
XML can be used to create documents that can then be used or enriched by various entities in a company, or to exchange data between two incompatible databases in a neutral and auto-documented form.
XML documents can be published to the Web, in WML for wireless phones, on paper... all from one form. A convenient use of XML is just to include new elements to an HTML document, like price or reference, that will be processed on the server side and rendered in HTML on the Web.
One of the most promising uses of XML is in e-commerce, where a set of tags can be agreed upon by several companies doing business together. They can specify item numbers, quantities, prices, customer numbers, etc..; When they receive such a document they can then extract this data and interface with their information system(s) to do order processing, warehouse, customer service, billing etc.
XML can also be used as the basic format to exchange messages between processes in distributed applications, allowing more flexibility in communications.