Difference between unlocking card decks and purchasing DLC deck packs

There are two ways of getting cards unlocked in the standard M:TG Planeswalker games - you either play through the single player game multiple times with each of the decks, which will eventually unlock everything, or you can pay for the deck unlocks. You can get the full listings of all the cards in the decks that come with the game at http://www.wizards.com/Magic/Digital/DuelsOfThePlanesWalkers.aspx?x=mtg/digital/d12/decks - this will show you not only the unlockables for the standard decks but also the expansion decks and scheme decks that are used in multiplayer.

The rest of the DLC out there (The Deck Packs and the Expansion Pack) are totally optional, but you can only get their decks and cards by purchasing them - they are not able to be earned simply by beating the game. Pack 1 adds "March To War" (a R/W Soldier deck) and "Ghoulkeeper" (a black Zombie deck). Pack 2 nets you "Dark Heavens" (Black & White, angels & demons) and "Forest's Fury" (a Green stompy deck). The expansion pack will get you three new decks in addition to new cards for the old decks.

Basically, what it boils down to is this:

There are a preset number of decks in the game. Each deck has a predetermined list of cards that can be unlocked specifically for that deck. As far as customization, you may only add/remove/exchange cards from that deck and its preset list of cards. For example, if you unlock a card in the Apex Predators deck, it can only be used to customize the Apex Predators deck. It can't be used in, say, Guardians of the Wood.

The DLC decks function in much the same fashion. Each DLC deck will come with its own list of unlockable cards that may be used to customize that particular deck.

A good list of each deck/cards unlockable for each deck can be found here.