In story terms a sequel is technically one that takes place after the original in the same universe, normally with at least some of the same characters, and of course a prequel is one published later, that takes place before the original.
Angels and Demons is a very interesting example. The book came out before the Da Vinci Code, which incidentally was the first sequel Dan Brown wrote, after several unconnected books. The 2nd book was filmed first, and confusingly when they came to film Angels and Demons, they changed it to take place AFTER Code, so the Angels and Demons movie is a sequel to the Da Vinci Code movie, whereas the books were the other way round.
I wish I knew what to call side-stories that take place alongside the events of a previous game, like the GTA IV Episodes, or Half Life Opposing Force, and Blue Shift.