Product Advertising API
Developer Guide (API Version 2013-08-01)

Using XML

So, what do you do with the XML response? The good news is that the Product Advertising API returns a well-defined set of tags for each kind of request. That means that you can use parsing mechanisms to pull out of the XML responses the data that you would like to submit in a second request, or, display on a web page. For instance, in the preceding example, you might parse the XML document to find FormattedPrice so that you could display the $29.98 on a web page that shows an item for sale.

XML is not only used to create requests and responses, it is also used to create a WSDL, which defines the XML tags.