XML schema is a way to define the structure of an XML file. There are three ways to do this via XSD schemas, DTD and Relax NG. However, XSD schemas are more common and this is the way that we define our BlogML specification as well.
BlogML has an XSD schema for each version that we publish as a part of BlogML release and our users can use it in order to make sure their BlogML files are valid. This is a main requirement to be able to use BlogML because BlogML tools are written based on this specification and don't work with an invalid file.