Halo 3 starts where Halo 2 left off: a source of some complaint about this previous game was the lack of an ending: Halo 3 does have an ending and what an ending it is.
The layout of the game more or less copies that of the previous Halos but there are many new weapon additions. Lots of Brute weapons: the spiker, the mauler to name but two, all having devastating results on whatever you fire them at. A third person view opens up when these weapons are in use, quite an effecive trick. You also have access to a gravity hammer for removing large numbers of the enemy at one swipe.
Vehicle chases are more common in Halo 3. The familiar Halo vehicles are all here as well as some new ones such as the Brute Prowler. A new addition to the series is that of "equipment." These deployable special items, such as the moon bubble which have a variety of innovative effects.
The continuing story theme has the Covenant on its way to Earth, attempting to activate floating space weapons known as Halos, which could destroy civilization. The Master Chief and the other Earth forces of the UNSC are hard on their heels with some new allies, the Arbiter. Enough details, but I promise that you are in for a feast of fighting and exciting effects. There are also more options as to the number of players taking part and the on line abilities. In addition to the four-player co-op action, you can also play competitive solo and team-based multiplayer matches with up to 16 players on 11 different maps. As in Halo 2, you can customize these game types, and there's more to customize and it's been made much more interesting than in Hero 3.
The file sharing options can be used to send screenshots and saved films, these being replays of action from any of the modes in the game. The game automatically stores the last 25 or so sessions, and you can choose to save them more permanently from there. You can edit them down to the more savable bits.
The game is technically impressive: it maintains a smooth frame rate throughout, and looks very sharp overall with lots of impressive lighting and other subtle effects, the visual is even better. The voice overs are clear and the comments realistic and believable with a lot of dialogue between comrades on both sides.
A good follow on in the series, sometimes as good, sometimes better, but all through of a pretty good game.