Translation:I want to buy a Christmas card.
Carta =letter Tarjeta =card
If it was Quiero comprar una carta de Navidad, It would be "I want to buy a Christmas letter", but if it was Quiero comprar una tarjeta de Navidad, It is "I want to buy a Christmas Card."
In Spain it is just "un christmas" for christmas card. Not sure if this is the same for latin america