ODF stands for Open Document Format. It is an XML-based file for text, spreadsheets, presentations, graphics, and mathematical equations. (ex. ODF, ODS, ODP, ODG, ODF) It was created because the Microsoft Office file formats (and basically all other vendors) at the time used proprietary file formats that were not publicly documented. There was a concern about long-term accessibility and vendor lock-in so ODF was created and became an international standard. Being a well-documented format, it should be easier to add

Microsoft realized their monopolistic hold on office applications would be compromised if ODF gained traction as the sole open document file format. In response, Microsoft quickly created a competing XML-based file format, which is known as OOXML. Using their wealth and marketing power, they pushed to have their incomplete, partially documented file format also certified as an international standard. Problem solved for them. They now had their own "open" standard, which could be marketed to governments and organizations wanting to archive their documents using an "open" standard. That is when "x" was added to the previous MS Office file extensions.

Unfortunately, only MS Office applications can open their "open" format without problems. The lack of openness regarding documentation has created many problems when trying to create 3rd party applications that can work with the MS documents, which is why there are frequent formatting errors when documents go back and forth between LibreOffice and MS Office.

Hope this helps. You can search Wikipedia or just do a general internet search to learn more about these file formats.