This article contains alot of canon insight as to who Snoke might be: http://starwars.wikia.com/wiki/Snoke

He is ca 7 feet tall but not a Muun or Plagueis, clearly, as he has a humanoid nose, no elongated skull, black alien eyes which is inconsistent with those of a Sith Lord, etc etc.

I reckon Snoke could be a powerful balanced force user, perhaps the most powerful, previously controlling (and now representing) groups like the Jal Shey or similar in the EU.