The origin of the Expanded Universe
Check out this cool introduction by George Lucas in one of the EU novels.
"It wasn't long after I began writing Star Wars that I realized the story was more than a singkle film could hold. As the saga of the Skywalkers and Jedi Knights unfolded, I began to see it as a tale that could take as least nine films to tell-three trilogies-and I realized, in making my way throught the back story and after story, that I was setting out to write the middle story."
"After Star Wars released, it became apparent that my story-however many films it took to tell-was only one of thousands that could be told about the characters who inhabit its galaxy. But these were stories that I was not destinied to tell. Instead they would spring from the imagination of other writers, inspired by glimpses of a galaxy that Star Wars provided. Today it is an amazing, if unexpected, legacy of Star Wars that so many gifted writers are contributing new stories to the Saga."
George Lucas