OOXML versus OpenDocument

When configuring Microsoft Word on a new machine, you will be prompted if you wish to have Office Open XML (informally OOXML) or OpenDocument (extension .odt for text) as the default format.

Choose the former if you want to be more Microsoft-centric and use all features of Microsoft's ecosystem, choose the latter if you need interoperability with applications like LibreOffice.

OOXML started out as an XML format for Excel and later extended to other applications such as Word.