Spartan Ops

Spartan Ops is the Halo 4 Cooperative Mode replacing Firefight mode from Halo: Reach and Halo 3: ODST. It will support up to 4 players and tie into the SPARTAN-IV Program and the Campaign of Halo 4.

Spartan Ops mainly revolves around Majestic Squad who will interact with the Chief during the campaign. The Remaining Covenant and Prometheans are the confirmed enemies in Spartan Ops. Eventually John-117 will meet up with the and this will somehow tie into the rest of the story.

Overview
343 Industries has confirmed that it will not be a paid DLC and be included free in the game. Seasons will be made up of Chapters containing five missions. Chapters will be released in weekly, free DLC's. Custom Loadouts are available for each Spartan Ops mission, in Spartan Ops you take control of squads such a Majestic Squad, and perform objectives like killing enemies or activating consoles. When it is finished, you are extracted by Pelican. You then gain your Rewards and move on to the next mission. One unlockable is the Battle Rifle. Once you finish a Chapter, wait until 343 puts up a new one. Eventually Season One will end and Season Two will be released. It is unknown if you must pay for the seasons after Season One.

Spartan Ops is said to take place six months following the events of the Halo 4 Campaign. It is believed you will be able to customize your Spartan in Spartan Ops.

Trivia

 * The title Catherine would suggest that chapter 3 is primarily about Dr Halsey although it could be refering to another character.