This article will contain a few small spoilers for Star Wars 7: The Force Awakens! If you haven't seen it, do not read this article unless you don't care if you learn about a few minor details from the movie, and either way, go see it as soon as possible, because it's extremely good!
So, many people have been speculating that Supreme Leader Snoke is the Sith lord Darth Plagueis (Darth Sidious' master). Although Sidious killed him in his sleep, many suspect that Plagueis was able to stay alive, due to his ability to manipulate midi-chlorians, and thus cheated death and escaped, presumably living in hiding.
There is significant evidence that Snoke is Darth Plagueis: they look somewhat similar, they both have great knowledge of the dark side, they're about the same height, and finally, the music of when Darth Sidious was telling Anakin the "Tragedy of Darth Plagueis the Wise" (possibly no coincidence that Kylo Ren refers to Snoke as being extremely wise), was almost identical to the music that played whenever Kylo and/or General Hux were consulting Snoke.
However, there are several problems with this theory, and that's what I'm going to be talking about. First of all, according to Andy Serkis, the actor who played Snoke (through motion capture), “He [Snoke] is a new character in this universe. It is very much a newly-introduced character. He’s aware of what’s gone on, in the respect that he has been around and is aware of prior events. I think it’d be fair to say that he is aware of the past to a great degree.”
One could argue that Plagueis was only ever mentioned (in canon), as a legend, or old tale, so he was never really "introduced," but really even that is establishing a character, and if he truly was a "newly-inroduced" character, then if you found out what his name really was in episode VIII (assuming it isn't Snoke), for example Hego Damask, (aka Darth Plagueis), then you would say: "Oh, Darth Plagueis, I wonder who that is. Hmmm, maybe we'll find out later in the movie..." In contrast, Darth Plagueis has already been established in Episode III, and there also is a novel about him, even though it technically isn't canon, but as long as it doesn't contradict new changes, it can be regarded as pretty legitimate.
Next is a quite obvious problem. Snoke was portrayed in the movie as a giant deformed 25 foot tall hologram of some sort of being. However, in truth he is 7 feet tall, and since muuns are of similar height, and Plagueis was a muun, many people have used this as more evidence for Snoke being Plagueis. One factor some people have missed, though is the fact that, to put it simply... muuns don't really have noses... also, they have tall, skinny, grey faces, and while Snoke does have a grey face, it is neither tall, skinny, nor nose-less.
Note: There are technically no canon images of Darth Plagueis, because he's never been confirmed to be shown in anything canon, but this is a picutre of him from the "Legends" part of his Wookiepedia page.
As I hope you can see, the two characters don't really look that similar, and although he could've had his face modified or something, it just doesn't really make sense.
Finally, Snoke and the Knights of Ren ARE NOT SITH, so unless Plagueis renounced the Sith and decided to make a new group of Dark-Siders, that doesn't really connect. In addition, many people have noticed how Kylo Ren isn't particularly good at fighting with a lightsaber. Also, he built his himself, and the other Knights of Ren don't seem to have any, which all leads us to believe that Snoke doesn't know how to create and use a lightsaber. That's why Kylo had to build his himself and train himself, (although he presumably knew some from when he trained under Luke), and it's also why he's the only Knight of Ren with one, and why Snoke stayed behind, directing everyone by hologram from who-knows-where, instead of helping with the fight. This is really, in my opinion, is what really seals the deal that Snoke isn't Darth Plagueis, because he, like all Sith, was (or is) a skilled lightsaber wielder.
I may be wrong about this, and at first I also thought that Snoke was Plagueis, but now, I can see how they're similar in some aspects, and in the end, they're just too different to be the same person.
And honestly, I know this may sound crazy, but not every new character in the Star Wars universe has to be related to an existing character in it, or secretly be another character that we've seen before!