The Rich Text Format (often abbreviated to RTF) is a proprietary document file format developed and owned by Microsoft since 1987 for cross-platform document interchange. Most word processors are able to read and write RTF documents. Unlike most of the word processing formats, RTF is human-readable (though machine-generated RTF such as produced by Word has a very high and obfuscating code-to-text ratio), and it's also human-writable: once you get used to it it's like a poor man's LaTeX (a very poor man's). RTF is also the basis for Windows Help (Winhelp) format.
See also Ratfink, a Tcl library for generating RTF files.
O'Reilly has an RTF pocket guide [L1 ], the only book I (EKB) know of that documents the RTF format. Microsoft documents RTF on 278 pages [L2 ].