RTF - Rich Text Format
RTF stores formatted text as readable markup rather than binary data. It is the old lowest common denominator between word processors, and it still works everywhere.
RTF stands for Rich Text Format. Microsoft published it in 1987 as a way to move formatted documents between programs that otherwise had nothing in common, and it succeeded so completely that almost every word processor ever written can still read one. Where plain text has no formatting and DOC had formatting nobody outside Microsoft could reliably decode, RTF put the formatting in the open.
How an RTF stores formatting
An RTF file is plain text. Open one in a text editor and you can read both your words and the instructions around them, because formatting is expressed as control words — backslash-prefixed commands mixed into the text. \b turns bold on and \b0 turns it off; \i does the same for italic; \par ends a paragraph; \fs24 sets the type size in half-points, so 24 means 12 point. Fonts and colours are declared in tables at the top of the file and then referred to by number.
That transparency is the format's strength and its weakness. A partly damaged RTF can often be salvaged by hand, and a program that only understands half the control words will still show your text with some of the formatting. But images are embedded as long hexadecimal strings rather than as compressed binary, so an RTF with a few photographs in it becomes enormous — routinely several times the size of the same document as DOCX.
There is no compression anywhere in the format, and there is no ZIP wrapper. What you gain in robustness you pay for in bytes.
What metadata an RTF carries
RTF has an optional information group, written as \info near the start of the file, and it is modest compared with DOC or DOCX. When an application fills it in you may find \title, \subject, \author, \keywords, \doccomm for comments, and \operator for whoever last edited the document.
Dates appear as structured groups: \creatim for the creation time and \revtim for the last revision, each spelling out year, month, day, hour and minute in separate control words. There is usually a \vern version number, an \edmins count of editing minutes and a \nofpages page count.
Two things make this metadata unusual. It is optional — many RTF files have no information group at all, so a metadata report on one can legitimately come back nearly empty. And it is readable in the file itself, so you can confirm exactly what is there by opening the document in a text editor, which is not true of any other formatted document format.
When to use RTF
Use RTF when you need formatted text to open in something you cannot predict — an old machine, a minimal editor, a system with no office suite installed. It is also a sensible interchange format when a document is mostly text and you want to be certain the recipient can open it.
Do not use it for anything image-heavy, where the file size becomes absurd, or for documents with complex layout, which RTF expresses poorly. For everyday editing DOCX or ODT are better in every respect; for a finished document, export to PDF.