.docx — Word Document
Document application/vnd.openxmlformats-officedocument.wordprocessingml.document
What a .docx file is
Microsoft Word's format since 2007. Despite appearances it is a ZIP archive containing XML — rename one to .zip and you can look inside.
How to open it
Word, Google Docs, LibreOffice Writer, Pages.
Worth knowing
Complex layouts and tracked changes do not always survive a round trip through a non-Microsoft editor.