Curse of Naxxramas is a solo adventure expansion for Hearthstone: Heroes of Warcraft. The expansion consists of five quarters (or wings) that each contain a series of bosses as well as two class-specific challenges. Once you have defeated the normal version of each boss, the class challenges are unlocked for that quarter, as well as heroic mode versions of each boss.
The Arachnid Quarter
The Plague Quarter
The Military Quarter
The Construct Quarter
Frostwyrm Lair (coming next week)