While I deeply sympathize with Foolestroupe about how plain text, maybe with a "simple" formatting commands, is plenty for a vast array of uses, I must correct the claim that the words "RTF" and "simple" could go together.

RTF is basically Microsoft Word format, but slightly more structured than Word native format. It has no published standard (although there are various semi-official docs about various versions of it available); from this, support for it in different programs varies highly, and it's not even close to easy to parse it fully. If anyone is interested, or wants to challaenge me on this, I'll dredge up some citations and URLs.