A gentle introduction to XML markup
xmln denotes a namespace
dc denotes the name of the namespace, the local part
http://purl.org/dc/elements/1.1/ is a unique identifier (URI)
This whole namespace thing become very useful when an XML document uses two or more XML vocabularies. For example, it is entirely possible (if not necessary) to have more than one vocabulary in RDF streams. There is one vocabulary used to describe the RDF structure, and there is another vocabu- lary used to describe the metadata. The example below is a case in point:
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/"> <rdf:Description rdf:about="http://www.AcronymFinder.com/"> <dc:title>Acronym Finder</dc:title> <dc:description>The Acronym Finder is a world wide web (WWW) searchable database of more than 169,000 abbreviations and acronyms about computers, technology, telecommunications, and military acronyms and abbreviations.</dc:description> <dc:subject> <rdf:Bag> <rdf:li>Astronomy</rdf:li> <rdf:li>Literature</rdf:li> <rdf:li>Mathematics</rdf:li> <rdf:li>Music</rdf:li> <rdf:li>Philosophy</rdf:li> </rdf:Bag> </dc:subject> </rdf:Description> </rdf:RDF>
Here you have two vocabularies going on. One is defined as rdf and assocaited with the URI ht- tp://www.w3.org/1999/02/22-rdf-syntax-ns#. The second one is defined as dc and associated with ht- tp://purl.org/dc/elements/1.1/. The namespace local parts are then associated with various elements a needed.
One final note. URI are simply unique identifiers. They often take the shape of URLs, but need not point to anything accessible over the Web. They are just strings of text.
The semantics of an XML document (think grammar) is an articulation of what XML elements can exist in a file, their relationship(s) to each other, and their meaning. Ironically, this is the really hard part about XML and has manifested itself as a multitude of XML "languages" such as: RSS, RDF, TEILite, DocBook, XMLMARC, EAD, XSL, etc. In the following, valid, XML file there are a number of XML elements. It is these elements that give the data value and meaning:
<catalog> <work type='prose' date='1906'> <title>The Gift Of The Magi</title> <author>O Henry</author>