RTF (Rich Text Format) is a formatting language. Unlike HTML, which is concerned with a document's structure, RTF is geared toward controlling the format of a document. So while HTML uses commands to indicate things such as lists, headings, and normal paragraphs, RTF uses commands to mark a document's font size, typeface, and style.
RTF was developed by Microsoft with the idea of interchanging documents between Microsoft Word and other word processing packages, although other word processing packages such as Word Perfect are able to read it as well. There is a filter called rtftohtml that can be used to translate RTF documents into HTML as well as to create new documents for the WWW.