I'll tell you in quick way so this topic can be closed
In the beginning there were only Jedi. But some Jedi left the Jedi order because they were more interestedi in the Dark Side...they became fugitives and ended up on an unmapped world where they were worshipped as gods by a primitive race called the Sith...they heired that name. They used the Sith to built temples, the Sith race were slaves under the rule of the Dark Jedi which later became the Sith themselves
The title Darth comes from a Sith called Darth Bane who created a new Sith order with a new rule "there can only be two" cause the Sith in their history were also known for destroying each other. From his name the title "Darth" came which is based on Julius Ceasar...cause the roman emperors used his name as a title too.
That's an EU explanation.
What we know from GL is that the Sith started out thousands of years ago, and once ruled most of the Galaxy, before turning against each other. Their actual origin is left unclear; GL's words might be taken to mean that the Jedi predate the Sith but also they may not.
GL does not seem to consider the possibility of the Sith ever having been a race, any more than there was ever a race called 'Jedi'. It's just a name.
I like EU which doesn't mess with the movies...I saw a comic where nobody knew how Jabba looked like, and the comic makers decided to make him a skinny, ape looking alien. That is totally wrong of course!
Owen Lars was once Obi-Wan's brother...they changed that now, but they didn't delete the story....now it's just a brother called Owen...which he almost forgot because of his training. I don't mind about all that! I think it's cool how they present the clone wars in comics and especially the clone wars animated serie! Now those are canon in my opinion.
Could be...The EU version is just larger and a more detailed explenation...I consider it canon, altho you don't hear from it in the movies...Like I said if it doesn't mess with the movies...I don't care. Then I even like it! But all old comics of marvel are not canon IMO. They just did what they thought would be cool and didn't care about the rest...Now Dark Horse looks at other facts so that what they write doesn't get in conflict with the movies...Not all is accurate tho. For example. For a long time after the battle of Geonosis Anakin uses a lightsaber that looks the same as the one from ep II which got destroyed...I don't think he built an exact same, but built the one which Luke got right after the battle of Geonosis...makes more sense. And in one comic he has his short padawan hair while in the other he has his long hair from ep III...they should have tried to make an evolution. Same with Obi-Wan and other that changed. Stories could also tell more about Padmé...what she did during the Clone Wars.