Uh... Dean went to Hell. The angel Castiel took him out of Hell because demons were trying to release Lucifer. Sam drank the blood of the demon Ruby to get more powerful, but it turned him into a blood junkie. Sam used his power to kill Lilith, which inadvertently released Lucifer. It's destined that the angel Michael will possess Dean and fight Lucifer, who will possess Sam. Castiel turned against his fellow angels to help keep Dean and Sam safe. It's the apocalypse, and Lucifer seems confident Sam will consent to being possessed on a certain point in time. Sam and Dean are trying to find a way to kill Lucifer, and Castiel is trying to find God, who apparently abandoned the angels at some point.
