The OpenDocument standard is a freely available, documented standard for creating not only documents but spreadsheets, presentations and other similar documentation. It has been voted as a standard by ISO but the paperwork is still to be completed. Its use is preferred over closed format documents such as MicroSoft Word as the applications that use it are permitted to understand all aspects of the document. In short, it's more compatible. These applications also tend to be free, the most notable being OpenOffice, and work on all sorts of computers. The format is also readily manipulated as it is a zipped-up collection of XML files (for non-techies; you can see its guts and fiddle with them using something other than a word processor).
-- Main.VikOlliver - 06 May 2006