What is a stereotype? Where can this be used? What is a stereotype? Where can this be used?
I think you are referring to a concept in UML modeling and not something associated strictly with XML.
In the Unified Modeling Language (UML) a stereotype is one way to extend the core semantics of the modeling language to express new things. For more information on extending the UML you should refer to the UML resources on this site, as well those on the OMG' s (who now owns the specification).