Lucas wrote the films. He wrote the novel for A New Hope. Everything else officially released has been written by other authors, but the content is OK'd by Lucas. This is what they call "Expanded Universe". Authors aren't just randomly writing Star Wars stories and publishing them.

For the novel adaptations of the other 5 films, Lucas gave the screenplay to the chosen author, who proceeded to turn it into a novel. Again, they are approved by Lucas before they get released.

Fan Fiction are the stories written by fans, not authors officially commissioned to write the books. These are the stories you see on message boards and blogs. I guess you could say they are "toloerated" by Lucas as fan adaptations of the saga, but that's all they are.

Edit: It appears that even the original Star Wars novel was ghostwritten by another author.