The Open Document Format is the file format for electronic office documents. It is standardized as ISO/IEC 26300. Implementors include OpenOffice.org and KOffice.
Please do not mix up ODF with Microsoft's attempt at destroying real document interoperability which pushes their own underdocumented and badly designed format "Office Open XML", a.k.a. ECMA 376. Here's why OOXML is a very bad idea: