Marc B. Wrote:Story lines from books, games, comics, etc.
For those still curious, to add to Marc's statements, George Lucas allowed and oversaw a tightly controlled expansion of the overall Star Wars universe. Unlike the hundreds of Star Trek novels written over the years (which if considered realistically would require the characters to have been taking part in starfleet missions for every day of their extended lives), Lucas's Star Wars Universe is kept to a strict timeline.
And the first stricture he put upon all the people involved, was that they could not write anything whatsoever that took place during the thirty years preceding Episode Four: A New Hope. As you've now surmised, this was so that he could conceive and create the prequel trilogy of The Phantom Menace, Attack of the Clones, and Revenge of the Sith.
As he did allow for books and comic books and audioplays to be created that picked up, literally, the day after the events of Return of the Jedi, it has always been supposed that Lucas never ever intended to do a third trilogy.
What makes the Star Wars EU so appealing to many fans is the fact that Lucas has a whole team of folks overseeing the continuity of it all, so that, with the exception of the novel Splinter of the Mind's Eye, no storyline, idea, concept, character, or event in any one form, conflicts with another.
Think about the fantastic continuity that Paul has created in The Adversary Cycle, and then multiply that by a power of ten. Thats the Star Wars EU.
And no, as unbelievable as it may sound, I'm not all that much of a Star Wars fan.