Shield Brothers is a new Strike coming with Destiny: The Taken King, which was the main focus of today's livestream from Bungie.
The level 36 Shield Brothers Strike takes place inside the Dreadnaught, which is Oryx’s Hive ship. Guardians will enter the ship in order to thwart a Cabal invasion and take on two of its Cabal commanders: Valus Tlu'ur and Valus Mau'ual. These two are essentially the Shield Brothers, as you probably figured out.
The two commanders have boarded the Dreadnaught, attempting to detonate its core, thus destroying the Guardians and the ship in the process. Players must stop them at all costs.
In Strikes such as Shield Brothers, Bungie is mixing things up a bit for players who replay the Strikes. Some of the encounters will shake up the dialogue and won't have the same set-up every time. Variety comes in the form of changing spawn points for enemies, and even different enemies. This will provide more incentive for players to replay these instances.
Strikes in The Taken King will offer various rewards when completing a particular strike, and the unique gear dropped will show other players in the game you have bested the particular boss of said Strike.
These bosses will also require players to use different tactics than they have in previous Strikes.
Destiny: The Taken King launches September 15 and you can have a look at some the Subclass information which was announced today, along with this nifty video explaining Artifacts.