An RTF file or Rich Text Format file is a way of storing a word-processor document so that it can be widely used.
An RTF file can be read by many different word processing programs, and looks the same everywhere (retaining emphasis, tables, indenting, etc).
Word, WordPerfect, Nisus Writer, Claris Works, and many others can all read RTF files and show the document looking much the same in all cases, on a Windows, Macintosh, Linux or Unix computer.