'OV' is short for 'Openbaar Vervoer' which means Public Transportation. The transportation card in the Netherlands is called the OV chipcard. The OV Chipcard is the payment method for public transport in The Netherlands. If you carry on an OV Chipcard, you don't need to buy a ticket. You load credit on the card in Euro and you can use all kind of public transport in The Netherlands!
There are 2 different kind of cards: a personal OV Card and an anonymous OV Chipcard. To be honest, it is impossible to live in Rotterdam without this card. We advise you to buy this card directly after your arrival at a train station!