A file format specifies how data is stored for a particular application. For example, a .docx file is an OpenXML formatted Microsoft Word document. Not all applications can read all file format; and in some cases an application may only be able to read parts of the file. For example, a application may be able to read the text, but not the formatting, of a file that uses a format other than its own.
OpenDocument Format (ODF) was originally developed to support the features and behavior of OpenOffice-based productivity suites and OpenXML was originally developed to support the features and behavior of Microsoft Office.
Microsoft Office now provides support for ODF and gives people using Microsoft Office the chance to save their Office files in ODF. However, because Microsoft Office and OpenOffice features aren’t exactly the same or aren’t implemented the same way, people who use unsupported features (see the differences links below) and save Office files in ODF might experience changes in how they can work with the file and sometimes see a loss of content.
The OpenXML format (.docx/.xlsx/.pptx) is the default format in all supported versions of Microsoft Office and, unless you have a specific reason to use a different format, it's the format we recommend using for your Office files.
To learn more about the Open Document Format (ODF) generally see OASIS Open Document Format for Office Applications.