In the simplest sense, XML Schema can be thought of as a means of dictating the format and content of XML documents. However, XML Schema's real power lies in the fact that it can be used as a platform-independent type-description language, where XML documents are then used to exchange data in accordance with those type descriptions.
Finally, there are two rules to consider when building XML Schemabased systems:
XML Schema schemas are best used to describe type systems first and document layout second.
When using XML within your own applications, make it a natural part of development to write schemas to accompany the documents. A document is useless without its schema.