The DOCX file type is primarily associated with 'Word' by Microsoft Corporation.
Office Word is an authoring program that gives you the ability to create and share documents using a set of writing tools.
This is the new format for Microsoft Office documents (2007). It is a combination of XML architecture and ZIP compression for size reduction. Microsoft promises the format will be open (published for anyone to use) and prior versions of Office, using components provided by Microsoft for Office 2000, XP, and 2003 will be available to perform document conversion. Remember that these are promises by Microsoft. We have yet to see the actuals.
ePub is now a standard e-book file format that is supported by almost* every e-book reading device include Sony Reader, BeBook, IREX Reader, iPad, iPhone and the upcoming Nook from Barnes & Noble's.
Other than hardware devices, you can also read .epub books on your desktop or mobile phone using free e-reader software like Stanza, Mobipocket, FBReader (for Linux), Aldiko (for Android) or Adobe Digital Editions.
Amazon Kindle uses a proprietary format (AZW) and cannot read ePub files directly though you can use the free Stanza desktop application to convert an ePub ebook into AZW (or even PDF), a format that your Kindle can understand.