XML Schema is a markup language that facilitates the declaration of simple and complex data types. The schema describes the types, elements and attributes. The language allows the author to create attributes that are optional or mandatory. For elements, the author can decide which are option, how many are required and in what order.

An XML Schema is a self validating document. Thus, one can use a validator to see if the document was structured correctly. If all elements of the document are valid, the whole document is considered valid.

Several more advanced features are also available in XML Schema, including: Inheritance, Inclusion (imports), Namespaces, Annotation, Local & Global Types, Attribute & Element Defaults and Identity Constraints.

The current version is XML Schema 1.0. The W3C is currently working on requirements for XML Schema 1.1.