What we know is that the Emperor is a man, he is old, and he is very powerful due to his Mastery of the Dark Side. That is all certain.
According to the "Revan" novel by Drew Karpyshyn, The emperor is also immortal (as in, cannot die to old age). The novel is a good read if you want to know more about the emperor (not all though, but some information).
(I've yet to play the jedi knight story through..)