Halo 3 is broken up into 5 different segments that simply accessed from the main menu. Since its arrival on the Xbox 360, a slew of new features and applications are available to the user. First off, your asked to sign in with your gamertag which will allow you to see your friends in game (press 'Y') and access other features such as saved films. I'll get into saved films a little bit later. One thing that is noticed is three new options for friends who play Halo 3. They are, Halo 3 File Share, Halo 3 Party Deatils, Halo 3 Service Record. I'll go over those a little bit later too. I'd like to first talk about Campaign mode.

Campaign in Halo 3 has gotten an overhaul not only is there new content (duh...) but Bungie has introduced, (what I think is the greatest thing to come to video games of this caliber) 4-Player Online Co-Op. Now Bungie, didn't want to replicate 4 Master Chief's so the Co-Op element is built into the gameplay. There are four different characters, (one of them being the Arbiter) and two other Elite (Sanghelli). You can read more about them here. The levels are put together very nicely, however I did notice some aspects from Halo: CE in which you go in a structure and have to come back out. In my opinion its kind of repetitive but I laughed amongst my friends as it reminded me of the original. One of the things you also notice, is the shear amount of additional enemies, and the AI difficulty that they bring. I notice that when you have more people playing Co-Op the enemies tend to be more difficulty, and can adapt to multiple players. In my normal progression I like to find shortcuts... Unfortunately, they were smart enough to find me and pin me down.
The next and probably the most important is Matchmaking, and of course we see some new features that I think are really beneficial to Matchmaking. The one I like the most is the ability to select the type of unranked and ranked playlist you wish to play. Before, specific playlists like Team Slayer, Team Skirmish, which are objective based games (Capture the Flag, Assault) were only ranked, and the Team Training (unranked) was the only playlist where you could access that unranked. So you would get a mixture of both. I personally do not like slayer games. Bungie has uniquely called the non-ranked games, social.
Another big modification to matchmaking is the ranking system. In Halo 3, each individual player receives a Service Record. A sort of experience ranking on how long they have played the game. You rank increases as you play more and more games, as you start out as a recruit, work your way through Private, Sergeant, Lieutenant, etc. You will also reeived a skill rating in ranked games. The idea behind the experience rank is to give credit to those players who don't cheat, and dedicate a lot of their time to the game. So to that affect, I've already seen people who are skill level 45 out of 50 in some playlists... However they are equal rank as I am, and I notice they play about the same as I do.
Custom games are pretty much the same, except there a host more options and game types to choose from. The most anticipated one is Infection, a gametype similar to "Dawn of the dead" or "Zombies". In Halo 2 many people played this gametype on the honor system and used some of the options in Halo 2 to make it a fun game. Now Bungie has brought those elements into an appropriate game type. Your able to select what the Zombies can and cannot do, what weapons they can carry, how fast they run and other options. Of course the usual gametypes are there too.
Halo 3 welcomes Forge, a map editor. No you cannot change the terrain but you can add and delete items. The interesting part, is that while you are doing all the editing you are able to have a game going on a the same time. I was to participate in one of the games with 10 other people and didn't find it too amusing; however I would use it to customize maps and make custom gametypes. Bungie has written up a nice "How-To" guide which explains Forge in great detail.
Lastly, and the one portion that I believe is the greatest element to any video game of this type is Theater. Theater is where you can retrieve, edit and share any game you've played in Halo 3. Whether its Campaign, Forge, Matchmaking or Custom, each game is recorded and your able to replay it back, make clips and take screenshots. Once your done your masterpiece you can then share it with everyone in the Halo: Universe. I think this portion is amazing and the fluidity of it all is amazing too.
So as you can see not only is the a gem for the Halo fans, there are some great things that even modest gamers will enjoy and look forward too.