Unlike SGML, an XML document does not have to have a companion DTD document. All XML parsers test to see that an XML document is well formed. The W3 Consortium requires parsers to verify that a document is well formed and to fail to render an XML document that fails that test. XML is a hierarcical set of entities. An entity is one or more elements.
Elements can contain one or more attributes and text.
XML provides a formal syntax that defines elements, attributes and text. In and of itself it really does very little. For that reason it is easy to define and specify. An element consists of an opening label, text, and a closing label. A special case of an element is an empty element.