Well,
here's the most recent specification from MS. (this is according to the Wikipedia article, I haven't checked myself)

The Wikipedia article lists 5 different versions, from 1.0 to 1.8 (some seem to have been left out, or not have specs...)

This is a HTMLized version of the 1.5 spec.

Here's a (google HTMLized copy) of some misc thoughts by an implementer or RTF

Some relevant quotes: "This document contains a few scribblings about things which don't seem to be covered in the RTF specification. "
and
"I have seen an RTF specification (non-Microsoft) which claimed that justifications apply to the last specified tab position. My experience is that the opposite is true. (The Microsoft spec seems to be silent on this point.)"
and
"Also, it has been reported that in Germany, "Standard" is used rather than "Normal" by some programs. "
and
"Style number 222 is special — it means "no style". (Why 222? I dunno.)"

Hope this is informative, and/or interesting... ;-)