Like
DTDs,
XML schema are a way to define the
elements of an
XML document. Unlike DTDs, however, XML schema use XML
syntax to
construct the definition.
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/.