Microsoft Word keeps three forms of a document:<ul>
<li>It keeps a version in its own internal RAM form with linked lists etc.
<li>It saves documents on your disk in a format the was not at least originally documented.
You could move such documents between Windows systems and Mac systems pretty well.
<li>Microsoft invented RTF as a documented format for an editable form for Word documents, but later abandoned it.

</ul>
It can also produce uneditable forms for the printer or PostScript or .pdf