For the free trial you don't need to buy anything. Just download it from the website. You only need to buy the game if you want to continue playing past the trial period.
If you do play to play long term, here's what you need to know:
Codes can't be reused. You'll need to buy them new.
Expansions don't work on their own. You need the core game and any previous expansions first.
There are currently three pieces to this game.
1 - The core game. This is the base game and the Burning Crusade expansion.
2 - The 'Wrath of the Lich King' expansion.
3 - The 'Cataclysm' expansion.
You technically don't need the last two right away but you will if you really want to play long term or if you want to play the Death Knight class or the Goblin or Worgen races.
Edited, Dec 8th 2011 4:03am by Karlina
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75 Rabbit/75 Sheep/75 Coeurl/75 Eft/75 Raptor/75 Hippogryph/75 Puk
75 Scorpion/75 Wamoura/75 Pixie/75 Peiste/64 Sabotender
51 Bird/41 Mandragora/40 Bee/37 Crawler/37 Bat
Items no one cares about: O
Missions no one cares about: O
Crafts no one cares about: O