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

Bosses:
Anub'Rekhan
Grand Widow Faerlina
Maexxna
Heroic Mode Bosses

Class Challenges:
Druid
Rogue

The Plague Quarter

Bosses:
Noth the Plaguebringer
Heigan the Unclean
Loatheb
Heroic Mode Bosses

Class Challenges:
Hunter
Mage

The Military Quarter

Bosses:
Instructor Razuvious
Gothik the Harvester
The Four Horsemen
Heroic Mode Bosses

Class Challenges:
Shaman
Warlock

The Construct Quarter

Bosses:
Patchwerk
Grobbulus
Gluth
Thaddius
Heroic Mode Bosses

Class Challenges:
Priest
Warrior

Frostwyrm Lair (coming next week)

Bosses:
Sapphiron
Kel'Thuzad
Heroic Mode Bosses

Class Challenge:
Paladin