The LyX folks would say it's a WYSIWYM (what you see is what you mean). Great tool. I wrote my dissertation and several papers on it, and it was a huge time saver, and a great way to focus on the content, not the form.
TeXmacs is not built on top of TeX. It is closer to a Lisp.
Yeah that's some pretty terrible naming. Why would you name something that is not based on TeX TeXmacs. Next you're going to tell me it isn't a Mac app.