The game requires a monthly subscription to play (paid in advance), although the first month is provided free with a purchase of the retail pack (unless you use the key from the retail pack to activate a free trial account, in which case your free month is lost...).
The prepaid cards are one way of paying for the subscription, the other being debit or credit cards. You do not need prepaid cards to play, but you do need a subscription. I am pretty sure (but I stand to be corrected) that World of Warcraft is PC only, there are no console versions for the Xbox or Playstation.
As far as the boxes go the artwork differs but the product is the same. Just make sure that if you are creating a new account (I am guessing you are) that you purchase the basic World of Warcraft pack and not the Burning Crusade expansion (its a green box and is marked pretty clearly) - the expansion requires the basic sofware to be bought seperately before it works. I wouldnt buy the expansion until you have tried the game for a while, it offers very few benefits until you are well into the game (other than two new starter races and locations)