Open XML

By default, documents created using 2007 Microsoft Office products will be based on new, XML-based file formats. Distinct from the binary-based file format that has been a mainstay of past Microsoft Office releases, the new Office XML Formats are compact, robust file formats that enable better data integration between documents and back-end systems. An open, royalty-free file format specification maximizes interoperability in a heterogeneous environment, and enables any technology provider to integrate Microsoft Office documents into their solutions.

Reference: http://office.microsoft.com/