Basic concept and implications of XML (and markup language approaches in general):
“Extensible Markup Language (XML) is a simple, very flexible text format derived from SGML (ISO 8879). Originally designed to meet the challenges of large-scale electronic publishing, XML is also playing an increasingly important role in the exchange of a wide variety of data on the Web and elsewhere.” (from W3C Page on XML)
Starter Links: W3C Page on XML | Wikipedia article | See also Michael Sperberg-McQueen’s “A Gentle Introduction to SGML” for an overview of the text-encoding ot text “markup” concept