OpenDocument is a FileFormat for word processing. It is an open standard using XML from the OASIS? group, and evolved from OpenOffice's original file format. It is the native format used by OpenOffice version 2.

You can sometimes learn a lot about something by seeing how it was created, and by who -- and that’s true here. OpenDocument was developed by many office suite developers, including those who develop OpenOffice.org, StarOffice, KDE’s KOffice, and Word Perfect (Corel). But some major users were involved, too, to make sure that their needs were met. Boeing was there; they have large, complicated documents, so their participation helped make sure that complex documents could be handled well. A Bible translation group also participated; they have lots of complex language needs, including multi-language documents and unusual languages; as a result, OpenDocument generally handles internationalization issues in a stellar way. They also allowed review by the public; [...]

(from http://www.dwheeler.com/essays/why-opendocument-won.html)

It should be noted that MicrosoftOffice will not support this format. They are relying on their own XML format document specification.

Much more detail is on WikiPedia:OpenDocument.