Sith. It happens.
That essentially sums up most of the EU.
To answer the OP’s question a little more directly, if you watch Ep. 1 without knowing anything else, it’s pretty clear from the dialogue, as I recall, that Sidious and Maul, the two bad guys, are Sith and are basically evil Jedi. This is the first mention of Sith by that name in a SW movie. I have no idea where in the EU the name Sith originated, or whose idea it was. I think everything mentioned so far comes from the comics, but I’m not sure.
Actually I think Lucas mentioned the Sith in the prologue of the original Star Wars novelization. No clue why he didn’t decide to use it in the movies until Episode 1.
But IF there are only two…
Darth Sidious has Darth Maul, who gets killed in I, THEN immediately in II, he has Darth Tyrranus.
Granted, at least a decade had passed but STILL, Tyrranus is pretty old & I don’t buy that he got all Sithy that late in life.
My understanding is:
All Sith are Dark Jedi, but not all Dark Jedi are Sith.
Hence, Darth Tyrranus may have been a Dark Jedi for some time before Sidious made him a Sith.
It actually made me think of Spock’s Brain;
“Sith and Sith, what is Sith!?”
There was a Seth Lord in the 1940 classic The Philadelphia Story. Is that relavent?
In a lot of the promotional material for the original trilogy, such as toys and collectors cards, Vader was often referred to as the Dark Lord of the Sith. But it was not explained what the Sith were until the mid 90s when the comics stories related here (Tales of the Jedi: Ulic Qel Droma, Darth Bane etc) were created. They were going to be about the origins of the Dark Jedi, and George Lucas suggested himself that they should involve the Sith, so it’s all considered canonical in that sense.
I had a cool poster with a image of Darth Vader made up of all sorts of smaller images from the film, little square with screen shots making a collage. It was great till I spotted the tagline was “Dark Lord of Sith” and not “Dark Lord of the Sith”
Now I’m not a fan of Star Wars in the same way I’m a fan of Trek, but just that ruined the whole poster for me 
Lost in Space III: The Revenge of the Smith
(To R2D2):“Come here, you collection of colliginous junk!”
“Party on, Wayne!”
“Party on, Darth!”
That’s how I understand it as well. I mean, think about it. Any Jedi can just decide to use the force for evil purposes, dominating minds to get free drinks, force choking guys who hit on their GF’s, etc…But only the evilest, bad-assiest of them all will be choosen by the current Sith Lord to be his apprentice. The other Dark Jedi, lacking a formal learning enviroment, will untimatley be forced to take jobs entertaining at kids’ birthday parties.
And just before tith.
The girls from planet Ibreek sure are cute but they have small tith.
Aren’t there also supposed to be Sith Witches? They are Darkside force users without the lightsaber chock-socky in the tradition of the ancient ethnic Sith. I don’t know if this was a concept that was abandoned or if it is something out of the comics.
That site made me want to start reading the old comics. I grew up in the country and was never able to get very many comic books growing up, so I just read all of the Star Wars novels I could get my hands on. How are the newer Dark Horse comics? Any good?
There were Force witches on Dathomir (and their pet rancors, too!) in the novel “The Courtship of Princess Leia”, but if there were actual Sith witches, I missed that corner of the EU.
Is it Tish that hits this shit Sith? It is.
The problem is that I don’t recall any point in the movie when anyone actually refers to “Darth Sideous.”
Re: mentioning the Sith in the prologue of the original Star Wars novelization.
Being curious, I took a look at the prologue and was a little surprised (only a little, because it had been 29-1/2 years since I had opened the book, after all, and didn’t expect to remember much) to see that Palpatine was cast as a patsy, not the dark master schemer: “Once secure in office he declared himself Emperor, shutting himself away from the populace. Soon he was being controlled by the very assistants and book-lickers he had appointed to high office, and the cries of the people for justice did not reach his ears.” No Sith mention.
So, pretty much all of this is EU. Is everything from the EU canon? Are KOTOR I and II canon? How much of it is contradictory or just plain crap?