Did you know that Luke Skywalker eventually got married? Or that Leia had twins with Han and trained as a Jedi? No? That's because it happened in the Expanded Universe.
When the world found out what George Lucas, Lucasfilm, and Disney had planned for 2015, one of the first questions that hardcore "Star Wars" fans raised was "What about the Expanded Universe?" It's a term mostly used by only the more dedicated of fans, but almost everybody has encountered the immense collection of books, comics, and video games at some point without even knowing it.
In short, the Expanded Universe is any officially licensed "Star Wars" story outside of the three live-action films. If it's got the "Star Wars" logo and tells a story, it is considered part of the Expanded Universe. That means television shows like "The Clone Wars," video games like "The Force Unleashed," and books like "Shadows of the Empire" are all a part of this aspect of the "Star Wars" universe.