All trains and most local public transport in the Netherlands have a unified fare and payment system that uses a contactless prepaid card called OV Chipkaart. While it is possible to purchase one-way disposable tickets at vending stations and, in some forms of transportation, from the driver, you will be charged a surcharge of 1 € per ticket.
If you plan to make multiple rides, it might be advisable to purchase an (anonymous) OV Chipkaart from one of the vending points (ticket machines in every rail station, ticket offices in some larger stations, customer service points in some shops) for 7.50€. This card can be charged with funds and used with the contactless readers in any railway station, in buses, trains, trams, and even some ferries.
When checking in, a lump sum is charged to your card depending on the mode of transportation. At the end of your journey, you need to check out again and then the difference between the lump sum and the effective fare is changed back to your card. For trains, you need a minimum balance of 20€ on your card to check in. For most local public transport around Delft, the lump sum is 4€. There is a mobile app through which you can monitor your balance, access your travel history, and apply for a refund in case you forgot to check out at the end of your trip.
More information about the OV Chipkaart can be found on https://www.ov-chipkaart.nl