RTF

RTF (Rich Text Format) is a format created by Microsoft and intended to be more broadly portable than their application/version/platform-specific file formats such as DOC and DOCX, but still proprietary to Microsoft (though other vendors' software can also read and write it).

Specs

 * RTF 1.0 spec
 * RTF 1.5 spec
 * RTF 1.6 spec
 * RTF 1.8 spec
 * RTF 1.9.1 spec
 * Some other spec versions in various formats including RTF itself

Other Links

 * Wikipedia article