Office 2003 XML Reference Schemas
Everything you every wanted to know about them is located here:
Microsoft offers open and royalty-free documentation and licenses for the Microsoft Office 2003 XML Reference Schemas. These Reference Schemas include SpreadsheetML (the schema for Microsoft Office Excel 2003), FormTemplate Schemas (the schema for Microsoft Office InfoPath 2003), WordprocessingML (the schema for Microsoft Office Word 2003), and and DataDiagramingML (the schema for Microsoft Office Visio 2003). The Schemas provide developers and representatives of business and government a standard way to store and exchange data stored in documents. Learn more about these technologies and how you can take advantage of them.
You can download the schemas and documentation here. Be sure to read Jean Paoli's Clarification of License Terms for Office XML Schema, for the scoop on icense terms.