The idea that I can describe XML adequately in a couple of paragraphs is ludicrous. The subject is complex enough that books are devoted so just the basic structure and usage. In my opinion, the reason for this difficulty it that XML is both flexible and extensible. So it can be used in almost any situation where data needs to be stored or transferred. Still, I'm brave (or stupid) enough to give a brief definition a try.

My two second description of XML has always been 'comma-delimited files on steroids'. My reason? The purpose of a CSV file is to store information so that another application can read/import it. This works fine, so long as the receiving application is aware of the structure of the incoming file. The best that CSV files can offer in this area is to place field names at the top of each column. So the possibility for problems of interpretation (or at least situations that require programmer-to-programmer discussions) exists. XML improves on this by applying a more rigorously enforced structure to the data. The order in which the data is physically laid out in the document is irrelevant, but each data field is named, so that it is easier to find. And when you combine XML data with DTDs or Schemas, the type and range of data are now subject to validation. Certainly a superior mechanism than CSV files.

For the most part, our use of XML will be to package up the data that needs to be send to the web service for processing and to receive back the results. As such, our XML documents will be relatively straightforward. But your complexity may vary, depending on the solution that you're trying to provide.