The Rich Text Format (often abbreviated RTF) is a proprietary document file format with published specification developed by Microsoft Corporation since 1987 for Microsoft products and for cross-platform document interchange.
Most word processors are able to read and write some versions of RTF. There are several different revisions of RTF specification and portability of files will depend on what version of RTF is being used. RTF specifications are changed and published with major Microsoft Word and Office versions.
It should not be confused with enriched text (mimetype "text/enriched" of RFC 1896) or its predecessor Rich Text (mimetype "text/richtext" of RFC 1341 and 1521); nor with IBM's RFT-DCA (Revisable Format Text-Document Content Architecture) which are completely different specifications.
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.