This brief excerpt of Frank P. Coyle's book explains that XML is "more than just tags," and says the power of XML is that it was designed to do only one thing and do it well.

Unlike SGML, XML does not require a document type declaration. However, a document type declaration can be supplied, and some documents will require one in order to be understood unambiguously. (A ...

This is the XML declaration. While it is not required, its presence explicitly identifies the document as an XML document and indicates the version of XML to which it ...