Literally spaking, the EU is any part of the Star Wars universe that, while BASED on the galaxy GL has created, is not actually something that comes from GL himself, or is directly derived from some such thing.
So the films, books, radio dramas and screenplays are NOT EU, because even if GL did not write the novels they are still directly based on his work.
The games and books and so on are all set in his universe but are original stories in their own right, and so EU.
As yerssot points out, the EU is not canon- though the two things are not necessarily the same. For example, parts of the novels are no longer canon, even though they are still not EU.
The EU is something that at times does conflict with the story that George Lucas started.
But most of the time it is just something to read, listen to, play, look at, and such that is Star Wars.
It just gives you something extra in the Star Wars Universe that is not the movies.
For some after seeing the movies they want more, that's what the books, comics, games, and such are for.
The Expanded Universe isn't for everybody though. But count me in.
Canon means it IS Star Wars and IS George Lucas' ideas. Meaning it was made by him and his company alone and it's from exactly what he wants.
Non-canon means it is Star Wars, but not exactly from George Lucas' vision. Like the Grand Admiral Thrawn from the book you're reading now. If that book was completely agreed to by GL himself then Thrawn would have been human, because the Empire wouldn't allow women or aliens, which Thrawn is, to work for them.
Yes. More specifically, when we say 'canon' with Star Wars we normally mean canon as pertaining to the films. EU has a canon of its own.
So canon is what is actually and indisputably true about that storyline. So, it is canonical that Luke destroyed the Death Star. That simply can't be argued.
But who designed it? The EU has had some ideas on that, but now he have it said CANONICALLY that it was the Geonoisans, so whatever the EU says about that is irrelevant.
The EU also has a plot about a prison planet around which the first Death Star was built. Nothing in the film yet contradicts that. But because it is EU, it actually has no offiical relevance to the films. So I could say that I don;t think it is true, and you can't say that I am wrong because an EU book says otherwise.