The title "Extended Universe" (aka "Expanded Universe") is one typically used in Star Wars fan circles to refer to the additions to the Star Wars universe made by the many books, games, comics, and other materials which have been produced based on the movie franchise. The extent to which these materials are considered canon varies widely from person to person, and from source to source. While everything is subject to being overriden by one of the films, certain additions to the universe can be found in countless books, to the extent that they are considered major parts of the Star Wars universe, despite never having appeared in any of the movies (eg Interdictor Cruiser, the Jedi Academy, Mara Jade) .

Regardless of the specifics, it is vital to understand the distinction between absolute Star Wars canon and extended universe in any discussion of the franchise. The movies are the only "true" canon of Star Wars, with the "extended universe" as a secondary canon, being accepted for all intents and purposes as part of the universe, but are subject to being overruled by the movies.

Some people have been known to refuse to acknowledge the "extended univserse" sources for having any value as canon, so here is a quote from actual Lucasfilm continuity editors explaining their position:
"Gospel, or canon as we refer to it, includes the screenplays, the films, the radio dramas and the novelisations. These works spin out of George Lucas' original stories, the rest are written by other writers. However, between us, we've read everything, and much of it is taken into account in the overall continuity. The entire catalog of published works comprises a vast history -- with many off-shoots, variations and tangents -- like any other well-developed mythology." (1)
So in other words, only the movie plots in their various forms are officially canon, but the extended universe materials are recognised as being part of the "overall continuity", and as such accepted as authoritative unless they contradict a true canon source.