According to the GI Article 'Spartan Ops' is structured like a TV Season, each week players gain access to a new CG Movie sequence that slowly unfolds the story, along with 5 objective based cooperative (2-4 Players) missions to play through. These missions will be new content offering unseen locations, events and challenges for that week. The missions will be replaced by the new ones every week, but the video will be archived to view any time.
This isn't paid DLC, its already on your Copy of Halo 4, its designed to stretch out across several months.
My Opinion: Well I didn't care much for Firefight but loved Invasion, I believe this is going to be amazing playing with friends/family. What I believe which is what will keep this going is people wondering what is going to happen next? I'm hoping later on they get a small team to be dedicated to 'Spartan Ops' to keep the CG movie sequence going throughout the games time.