From the standard:
"Its purpose is to provide a consistent set of definitions for use in other specifications that need to refer to the information in a well-formed XML document [XML].
It does not attempt to be exhaustive; the primary criterion for inclusion of an information item or property has been that of expected usefulness in future specifications. Nor does it constitute a minimum set of information that must be returned by an XML processor. "
An infoset is a hierarchy (or tree) of items with named properties.
Each property can have a simple value (e.g. a name, text, URI, etc.), other items as its value, or lists of either simple values or items.