I am wondering what is the different between halo 3 and halo 3 odst. Is it the same missions or is it more weapons or more forge maps
It has a new campaign, firefight in it, and all of the Halo 3 Map Packs on a separate disc.
ODST is better
> ODST is better
True. In fact, ODST is by far my most favorite campaign among all Halo games.
Anyway, ODST has a brand new campaign that takes place on Earth during the same time as Halo 2. As the name suggests, you do not play the Master Chief or another Spartan, but an ODST. To be precise, you will be playing as five different ODSTs during the course of the campaign.
The campaign is, to put it in a nutshell, brilliant. It’s a tad short (you can play through it during a single afternoon, provided you don’t go hunting for achievements and secrets), but far darker and more “personal” than any other campaign.
ODST doesn’t have multiplayer of its own like other Halo games. Instead it introduces Firefight (which unfortunately lacks matchmaking support, so you have to get some friends together or play alone), and it comes with a second disk containing the complete Halo 3 multiplayer part including all map packs. So, if you have Halo 3 and none or only some of the map packs, ODST is a bargain. In fact, ODST was released as Platinum Hits edition some time ago and costs only 30 bucks (if that much at all), so it’s a bargain in any case.
Short answer: it is another game, it just happens to have “Halo 3” on the title.
But if you have to know, google is your friend.