Ooo is Earth following a nuclear holocaust that mutated everything (The Great Mushroom War which they frequently reference). Somehow the nukes were infused with magic or something, and that's what created the Lich. The Ice King used to be a human called Simon. In the immediate aftermath of the war, he found Marceline and took care of her. Simon, however, had also found the magic crown, which at this point is the only thing keeping him alive, but it's also driving him insane with it's chaotic ice powers (and causing him to slowly lose his memories). [url=http://adventuretime.wikia.com/wiki/Adventure_Time_with_Finn_and_Jake_Wiki]The wiki can probably answer your questions a bit better.[/url]