The XML Information Set (Infoset) is an attempt to define a data model complete enough to represent anything that can be stored in an XML document. The infoset defines a number of “abstract” building blocks, such as elements, attributes and characters.
It’s probably a good idea to see the infoset data model as the basis for the entire XML universe. The XML 1.0 specification simply describes a way to store infosets as byte streams, and standards like DOM and SAX provide programming interfaces.
Or in other words, everything beyond the infoset proper are just implementation details.