Together with the launch of Office 2007, Microsoft also presented a new kind of file format for the programs that were part of that office suite. This new format is known as OpenXML (OOXML), and its main extensions are .docx, .xlsx and .pptx.
Make OpenXML compatible with Office
The main purpose of the appearance if this new format was to make sure that the files created with Word, Excel and PowerPoint included the data of all the elements that were included in the documents (graphs, diagrams, images,...).
The problem with the appearance of these formats was that Office XP, 2000 and 2003 weren't capable of recognizing them, and that was the reason behind the development of Microsoft Office Compatibility Pack, a patch that completely solved the problem.
If you have a version of Microsoft Office that is older than 2007, and you need to open one of the documents created using this format, don't think about it twice and install Microsoft Office Compatibility Pack.