What is XML Schema?

XML Schemas express shared vocabularies and allow machines to carry out rules made by people. They provide a means for defining the structure, content and semantics of XML documents.

DTDs have several limitations, one of which is the fact that they are not written in standard XML data syntax. This means, for instance, that while it is quite possible to write an XSLT transform to document an XML Schema, there are far fewer tools to process DTDs

XML Schema also offers several features which are urgently required for data processing applications, such as a sophisticated set of basic data types including dates, numbers and strings with facets that can be superimposed - including regular expressions and minimum and maximum ranges and lengths.

An XML Schema: