This is probably an easy one to answer, but I was just curious, the Wikipedia articles weren't exactly insightful on how they might differ. I was reading some stuff on the Fatimid Caliphate and Ayyubid Sultanate when I realized I never even questioned why there were two different names for Islamic political entities. Any information here would be nice, thanks in advance.
A caliphate is, rather obviously, ruled by a caliph. The caliph was originally, and remained in theory, the supreme religious and political leader of the Islamic world and the Muslim faithful (umma). That's the theory, at least; the system broke down into rival caliphates, as you've identified vis a vis the Fatimids and Abbasids, both of whom claimed leadership of the umma. A sultan is an independent (in fact if not in name) ruler without claim to religious authority. One was not necessarily superior to the other. The Seljuk sultans, for instance, effectively ran much of the Middle East while paying lip service to the now-powerless Abbasid caliphs.