Is the normal Oblivion the same as Oblivion:Shivering Isles?

I'm getting my new xbox sometime soon and my friend got me involved in Oblivion but then he told me about Shivering Isles, should i buy both or just Shivering Isles? I need help fast because I'm at the store now deciding and the guy working said, "I have no clue." and he's standing here scratching his head; lol not knowing im doing this post but help is much needed thanks.