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MSDN Magazine article on Open Packaging Conventions (OPC)

msdn-opcThere's a good article on MSDN about the Open Packaging Conventions that are used by OpenXML and XPS. 

OPC provides an open standard container technology that you can use without having to code your own custom binary container files. And it supports a number of enhanced features, including content addressable URIs, MIME types, relational structuring, and authentication and validation.