The description of docx format expansion
The docx file is designed in order to store the layout of the text files, created and opened by Microsoft Word 2007 and later. In short, it is a collection of the plain XML files, compressed into a single zip-archive. It contains the standard text, objects, styles, formatting and images, which compound the complex docx object. In order to check it up for yourself, open any docx file with your favorite archiver (WinRAR, WinZip, 7-Zip) and reveal the compound structure of the file object.
In 2006 and prior to this date the predecessor of docx format (doc) could use only binary files for the data storage, and that severely limited the functionality of text editors, applied those times. Everything changed with the appearance of docx in 2007. After that the MS Word native format achieved its contemporary shape and was warmly welcomed by the ordinary users and expert audience.