How to get the card back for beating Heroic mode in the Curse of Naxxramas

Fireside Gathering.
You need to challenge a player on the same subnet as you 3 times while at least one additional player (3 total) is on the same subnet. There are sometimes real life gatherings where people meet to unlock this card back, but it also works with specific software or network emulation tricks.

No, you'll just earn 6 cards (1 legendary and 5 'commons').

Cards you'll earn :

Haunted Creeper good for a zoo deck (it's a deck with low cost cards and very agressive) you'll earn this if you beat Anub'Rekhan in normal.

Nerub'ar Weblord good for counter battlecry, you'll earn this if you beat Faerlina in normal.

Nerubian Egg it'll combo with a solfurie or an Argus, because if you destroy it, you'll earn a nerubian (it's a 4/4 cards), you'll earn this if you beat Maexxna in normal.

Maexxna she destroy any card she attacked, it can be usefull if you play Anduin Wrynn because he can heal Maexxna. You'll earn this if you beat all the 3 boss in normal and you'll unlock heroic and druid and rogue class challenges.

Poison Seeds : destroy all monsters on the battlefield and give treants 2/2 for each destroyed monster. Can combo with a druid token. Unlock when you finish the druid class challenge.

Anub'ar Ambusher a 5/5 for 4 mana but with a Deathrattle (Return a random friendly monster in your hand) can combo with Si 7. Unlock when you finish the rogue class challenge.

You can find many others information on the wiki : http://hearthstone.gamepedia.com/Curse_of_Naxxramas