Pretty much everybody knows what XML is: it is a structured, machine-readable text format for representing information that can be easily checked for the “grammaticality” of the tags, attributes, and their relationship to each other (e.g. using DTD’s). This contrasts with HTML, which can have elements that don’t close (e.g. >foo>bar rather than >foo>>bar>) and still be processed. XML was only ever meant to be a format for machines, but it morphed into a data representation that many people ended up (unfortunately, for them) editing by hand.