Doc and Docx are two basic file format or extension supported by Microsoft word. One of the significant differences between the two file extension is that Doc is the only format for files typed on Microsoft word 2003 or older versions of the software, but in 2007 we have Docx which is also a file format for Microsoft Word.
However, the problem with Docx files is that Microsoft word 2003 and the older versions cannot open or access it unless you update your Microsoft word to 2007 or other newer versions of Microsoft Word. In other words, Docx can be used on more unique versions of Microsoft Word, while Doc cannot be used on newer versions of Microsoft word unless compatibility package is used with Microsoft word 2003.
Another difference is that Doc uses the binary file to store documents, while Docx works like a zip file and contain XML files that are attached to the document.