Read
http://www.ov-chipkaart.nl/?taal=en to learn about the new OV Chipkaart. It's kinda like the Oyster card in London, or the Octopus card in Hong Kong if you are familiar with those. You load it up with cash, and touch in and touch out of the metro or tram.
It would be great if they go the extra mile like Hong Kong and make it work in shops too!