There's two stored value cards: one is called ez-link, the other is Nets Flash Pay. Both are stored value types; the functionality is identical, they're different just to allow for competition in the market. There's no weekly/monthly commuter type ticket similar to what one commonly finds in the US. AFAIK there's no longer a tourist pass although if memory serves there had been one many, many years ago. In any case, the reason for purchasing either of these stored value cards is because using them to pay your bus or MRT fare is cheaper than paying cash (for bus) or purchasing a single ride ticket (for MRT/LRT). The other reason is that a discount kicks in if you need to ride on multiple services (any mixture of bus or MRT/LRT) in order to go from point A to point B within I think a two hour time period, something not possible if you pay cash or use the single ride ticket.

One other thing: you will need to purchase the card, then add value to it. Under that circumstance I'd advise you to load as little value on the card as possible (keep refilling small amounts, eg S$5 or $10) and just keep the card afterwards as a souvenir. Yes you can turn it in for a refund on the balance but you'll never get back the original purchase price. Easiest refill points? Any MRT station. BTW if you need to run down the value, certain local merchants will take it, eg 7-11.

To find out more, try googling 'Transitlink Singapore', ez-link, or Nets Flashpay.