The reveal of a new game mode called "Spartan Ops" will be shown on June 4th (10 p.m. EST) during a show of Game Trailers by Spike TV.
What we know of Spartan Ops so far is that it is a four-player co-op mode that follows a group of Spartan IVs called Majestic Squad. Unlike Firefight, this mode will be objective-based and have five new free maps released every week along with a CGI movie to explain the story.
Information from n4g news.