The XML schema for Google Product Review Feeds defines the acceptable structure of your XML feed. XML schemas use the .xsd file extension.

XML schema. An XML schema gives guidance on how to structure the XML feed and uses the .xsd file extension. The XML schema for Product Ratings is published here ...

When creating your data feed, remember to save your file in one of the supported file types with the appropriate filename extension: Supported filename ...

There are a number of tools available to help you validate the structure of your XML files: W3C list of XML schema tools · Altova XMLSpy XML Editor · oXygen XML ...