The Sith were a race of fanatical dark-jedi who, in their following of the dark path, managed to eventually pervert themselves to the point of becoming a different race. They eventually scurried off, having found a planet all their own, which they would use as a base of operations to hone their dark skills. The Sith were already dead by A New Hope because, incidentally, the entire Sith race was exterminated when their home planet exploded. (The author is not sure of the exact chronology, but I believe that this had already happened by "Episode I" as well, which doesn't explain why or how Darth Maul got there.)
Also, Darth Sidious is NOT a Sith, he is obviously Senator Palpatine in a black cloak, they're the same actor for Christ's sake! As for Darth Vader, he attained the honorary title "Dark Lord of the Sith" by the being the meanest mo'fo around, not by actually being a Sith. Currently the Sith solar system is un-enterable, due to a heavy ring of planetary debris that surrounds the system. I gleaned this knowledge (although it isn't verbatim) from the Star Wars Universe A-Z.