OpenXML is the document forward used by Microsoft Office since Office 2007 on Word, Excel and PowerPoint. It carries an extra ‘X’ at the end of its extension i.e. docx, xlsx, pptx.
Microsoft also made available a SDK for developers to manipulate the documents without relying on Microsoft Office. Those who follow my blog would know back in 2008 to 2010 I wrote a few post on working with Office documents on the server side without depending on Microsoft Office.