Overview
Though there are a number of schema languages available, the primary three languages are Document Type Definitions, W3C XML Schema, and RELAX NG. Each language has its own advantages and disadvantages.
This article also covers a brief review of other schema languages.
The primary purpose of a schema language is to specify what the structure of an XML document can be. This means which elements can reside in which other elements, which attributes are and are not legal to have on a particular element, and so forth. A schema is somewhat equivalent to a grammar for a language; a schema defines what the vocabulary for the language may be and what a valid "sentence" is.
Read more about this topic: XML Schema Language Comparison