What exactly IS the Magisterium?

The Pope and the bishops are the magisterium?

The reason I ask is because I often hear of how the Vatican is steering away from traditional teaching by the Magisterium.

Much will depend on WHO you are hearing this from. 
There are those divisive folks who believe Pope Francis is teaching a heresy in AL. 
He is not. 

The promise of Christ cannot fail: when the Church presents some doctrine as definitive or final, it comes under this protection, it cannot be in error; in other words, it is infallible. This is true even if the Church does not use the solemn ceremony of definition. The day to day teaching of the Church throughout the world, when the Bishops are in union with each other and with the Pope, and present something as definitive, this is infallible. (Vatican II, Lumen Gentium # 25). It was precisely by the use of that authority that Vatican I was able to define that the Pope alone when speaking as such and making things definitive, is also infallible. Of course, this infallibility covers also teaching on what morality requires, for that is needed for salvation.

pianistclare Something to consider. Christ's promise that the gates of Hell would not prevail against His Church was aimed at His universal church, not the Roman church in particular. Christ's One Holy Catholic and Apostolic Church is made up of 24 canonical sui iuris churches of which Rome is only one, albeit the largest in terms of population. The Roman Church is not above any of the other Catholic Churches. They are equal in all things. We all have the Bishop of Rome as our head insofar as he declares anything dogmatically binding. Outside of that, Eastern Churches have their own Bishops and Metropolitans for the administration of our canonical churches. My point, that I think is worth consideration, is this. 23 of these 24 churches could fall completely into error or go 100% apostaste, and yet Christ's promise would remain true so long as one Church remained. So the entirety of the Roman Church could fall into error, and this would not be against His scriptural promise to always be with His Church or that the gates of hell will not prevail, as Catholicism would still, theoretically, be alive and well in the Ruthenian Church, in the Melkite Church, in the Maronite Church, in the Syro Malabar Church, in the Syro Malankara Church, etc. I know that the idea that Rome is the be all and do all of Catholicism is common in the West, as we aren't taught about the Eastern Catholic Churches in RCIA and are typically taught that Roman is synonymous with Catholic. But the Catholic Church is bigger than Rome.