Although both RDF and XML are used for codifying information, there are some fundamental differences between them which can be summarized as given by an information sheet issued by Semaview.com (not in business anymore):
XML is a tree, RDF is a set of triples;
XML is ordered, but RDF is not ordered;
RDF uses XML as one serialization mean for its data model built upon triples;
RDF is easier to subset;
RDF is easier to query using SPARQL.
RDF allows some abstraction from the document syntax;
RDF is more complicated and requires some planning;
XML is syntactic, RDF is semantic;
Not all projects need RDF.