GRDDL is a method for embedding machine-readable data into an XHTML document. GRDDL does not define any specific way of embedding this data, instead it uses XSLT for extracting the information from the XHTML document and producing RDF. Using this method, the machine-readable data within the document can use almost any representation, because GRDDL applications will always be able to transform this representation into RDF by transforming the document with the XSLT associated with the document.
XMLArmyKnife - Experimenting with EmbeddedRDF and GRDDL Support(About) Embedded RDF is a method of embedding (a subset of) RDF within XHTML and HTML documents. A simple XSLT transformation can be used to extract the RDF from within the document.
A related and more generalised technology is GRDDL which defines how to associate transformation algorithms (i.e. XSLT stylesheets) with XHTML profiles or microformats so that there's a clear mapping from embedded metadata into RDF.
I've been experimenting with adding support for both of these technologies in the XMLArmyKnife SPARQL query service. This provides a means to directly query RDF embedded in XHTML documents.