XML Basics [PMCP Tutorial]
XML data is exchanged in XML documents, and the form of the data in a particular XML document is described in one or more XML schemas. In other words, the the format of the data in an XML document is defined in one or more XML schemas. Just like HTML (web) pages, XML documents are comprised of text and data that is contained between items in a hierarchy of 'tags.'
One can define an XML schema in a fashion that ranges from loose to very tight. For example, the "address line" in an XML schema that defines an address could be defined to permit an alpha-numeric character in any position, or to require up to 5 numerals plus a space before any alphabetical characters.
The basic consideration is how closely the schema designer wants the schema to be 'mapped' to the data being exchanged.
While XML data is exchanged in documents, how those documents are exchanged is effectively up to the user -- or the designer of a specification that employs one or more XML schemas.