Halo 3 ODST also brings new light on the events before the story of Halo 3, and even these few details are not essential to the understanding of the Halo story as a whole, we like to find out what we reserve this new chapter.
Among the novelty, I also cite multiplayer "Baptism of Fire" who gets in outline the "Horde" mode of Gears of War 2: 1 to 4 players struggling to survive on a card drawn scenario against relentless waves enemies until exhaustion of number of lives. A great moment of bravery !!
Halo 3 ODST is ultimately a big additional chapter in Halo 3, but sufficiently long and provided on the single player mode (provided minimum starting difficult ...) to be played independently if you have never tried an episode of the saga. Baptism of Fire Mode and the presence of a DVD including the full Halo 3 multiplayer with all of the included download cards plus 3 new maps will convince you, if not already, you embark on the adventure.