Looking at many reports and descriptions of anal orgasms it seems that there are several different types. Prostate-triggered are very common, and result in either an "ordinary" ejaculation response orgasm or an orgasm with a different ejaculatory response. In most cases there's a qualitatively different feel, and in some cases the men report not having the same feelings of a refractory period afterwards. It's possible that the female prostate (G-Spot) can be stimulated via anal activities as well.
There are also "anally-triggered" orgasms, which are pretty much ordinary orgasms that are triggered from anal play, probably because of the close proximity of the genitals and genital nerves. These usually happen combined with some genital play at some point during the session, if they happen with concurrent genital play I term them "anally-enhanced orgasms".
Last (and most elusive) is the full anal orgasm. From reports and descriptions it sounds as though this is a response of the rectal-anal area, possibly including the support muscles, and not involving the genitals.
I'd like to research this more, but I"m not exactly sure how to proceed - it would be a qualitative study, I know that much, but I'm not sure quite how to format the questions or find participants yet. Above that there's always the time question...