So today is Halo 4 news day, evidently. In addition to the details on Halo 4's Special Edition release, Microsoft additionally revealed new details regarding Halo 4's multiplayer suite. Most notably, the Spartan Ops mode.
Spartan Ops is a story-based multiplayer mode for up to 4 players that takes place aboard the UNSC Infinity, the ship that acts as Halo 4's primary multiplayer hub. The missions that take place in Spartan Ops are essentially an offshoot of the main Halo 4 campaign, with new missions released on a weekly basis that flesh out the story of the Infinity, its crew, and a new team of Spartans. These missions will allow players to "explore the corners of Requiem in these objective-based missions, and help uncover the secrets of the mysterious Forerunner world."
This mode comes in addition to the previously revealed War Games mode, which serves as Halo 4's competitive multiplayer option.
I expect we'll be hearing a whole bunch more about Spartan Ops and War Games come E3, since, you know, that's kind of what E3 is meant for.