An XML Schema defines the structure and vocabulary of an XML document. The W3C developed XML Schema to create a more powerful and expressive XML validation language than DTD.
The Document Type Definition (DTD) describes the vocabulary and structure of an XML document. The term vocabulary refers to the elements and attributes valid to the XML document. The term structure refers to how these elements and attributes are combined with each other.
XML stands for Extensible Markup Language. XML is a language that lets you create markup languages. That is, a language that uses tags to describe other languages (e.g. HTML, TeX or MathML). This page is part of a series of reference blog entries to understand XML and related technologies.