Halo 4's Spartan Ops is a feature new to the upcoming game, and replaces the old Firefight mode from Halo 3.
Spartan Ops is a basically series of cooperative missions that's expanded through the release of five chapters, or missions every week after the game's launch. Each incremental add-on is a cinematic-driven addition to the game designed to be played with two or more players.
Halo 4's Spartan Ops shares a number of similarities—and differences—to Call of Duty's Spec Ops. Spec Ops is a cooperative game mode consisting of both objective and survival-based missions, spanning across 30 individual missions varying in difficulty. More difficult missions are unlocked in Spec Ops as you progress through the easier ones.
In contrast to Spec Ops, Halo 4's Spartan Ops is a cooperative mode which you can play straight out of the box without having to unlock. Rather than playing through a series of independent, disjointed missions, the missions you play through in Spartan Ops make up a big part of the on-going story in Halo. Additionally, five new missions will be released for Spartan Ops each week after the game's release.
In other words, you'll be getting a big bang for your buck in Halo 4 without having to shell out money for very little content. It's not unlike how Halo Reach's challenges are updated daily and weekly.
Each chapter, or mission, takes roughly 15 to 25 minutes to complete depending on the difficulty you play on. This adds up to two hours of gameplay for each weekly dispatch. Each mission is story-driven, and pertains to the accompanying cinematic.
Halo 4 is the first triple A title to offer story-based content in weekly increments, and may well set a standard for how content is delivered in the future. Episodic content may have been attempted in the past by Valve with the Half-Life 2 episodes (which have yet to conclude), but it's never been done well—until now.
And the best part about Spartan Ops is that the first season will be free—or rather, included in the price of the initial purchase of Halo 4. It doesn't get much better than that.