you can buy the cards at the little kiosks at the metro stations but they are only open during the day and close around 5pm. Having a card gives you a discount every time you use the metro and you can also use it on the buses as well (also discounted). So its really worth getting one. When you swipe your card on the metro or bus it tells you how much money you have left, and you can top up your card at the metro ticket offices.