A document description language (or MarkupLanguage), quite similar to SGML or TeX. Used by MicrosoftWord, as well as other word processors,
such as WordPerfect and TextEdit on MacOsx.

The specification for the format can be found at: http://msdn.microsoft.com/library/en-us/dnrtfspec/html/rtfspec.asp

Associated extension is .rtf

Some software projects have a requirement to exchange data with MicrosoftWord. I have encountered at least two such projects that employed RTF as an interchange format.  It is reasonably easy to write a portable Unix program to generate RTF.  

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Some people have observed that RTF loses some formatting when converting from MicrosoftWord, particularly for word documents that are precisely formatted or improperly formatted. Perhaps MicroSoft recommends it because of the business argument; RTF is annoying enough to compel some people to abandon their RTF-aware word processor and buy the full MicrosoftWord program. 

* A side-effect of being a near-monopoly. Many suspect they keep MS-Access crippled for similar reasons: to not cut into MS-SQL-Server sales.

If you work in a field where MS Word isn't totally dominant some "universal" document type is needed, so the choice is RTF or HTML (HyperTextMarkupLanguage). RTF at least has ''some'' formatting.

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AppleComputer adopted RTF for use in MacOsx.  For instance, the TextEdit application supports RTF.  

TextEdit also supports an .rtfd file format which is really a file package (a directory containing an RTF text file and one to many EPS and TIFF image files). This allows a document to carry its own images without (a) altering the file format or (b) referring to images stored in another directory or server.

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''I can't say I've played with it much, but it seems to lack the PowerOfPlainText.''