The OV-Card is little card with a chip used for paying public transport in the Netherlands. In case you finish your studies in ten years or less it is free. There are two ticket variants students can choose from:
Valid on working days (from Mondays 4 am to Saturdays 4 am, except on holidays and from July 16th 4am till August 16th 4am)
Valid on weekends and during public holidays (from Fridays 12pm till Mondays 4am).
For journeys outside the explicit ticket times, there is also a discount of 40% for train travel and a reduction for traveling by bus, tram or underground with a previously bought pink multiple transfer ticket.
Tipp: You should request the OV-Card at least three months before your study starts. You can find the request form at duo.nl.