XML Schema Languages

1.DTD Data Type Definition

Basic.

2.XML Schemas (XSD)
Widely used

3.RELAX NG
RELAX NG is a schema language for XML. The key features of RELAX NG are that it:

is simple
is easy to learn
has both an XML syntax and a compact non-XML syntax
does not change the information set of an XML document
supports XML namespacezs
treats attributes uniformly with elements so far as possible
has unrestricted support for unordered content
has unrestricted support for mixed content
has a solid theoretical basis
can partner with a separate datatyping language (such W3C XML Schema Datatypes)


4.Schematron
The Schematron differs in basic concept from other schema languages in that it not based on grammars but on finding tree patterns in the parsed document. This approach allows many kinds of structures to be represented which are inconvenient and difficult in grammar-based schema languages. If you know XPath or the XSLT expression language, you can start to use The Schematron immediately.