, XML schema
are a way to define the elements
of an XML document
. Unlike DTDs, however, XML schema use XML syntax
As of this writeup, schemas are at the W3C Working Draft stage and are not widely implemented. However, it is expected that they will replace DTDs in the future.
The W3C formal specification (work in progress) lives here: http://www.w3.org/TR/2001/WD-xmlschema-formal-20010320/.