The Nederlandse Spoorwegen (NS) operates most rail connections in the Netherlands.
All connections to and from Amsterdam and Schiphol Airport are run by NS.
NS used to have a monopoly on all rail connections, but nowadays some (mostly) minor connections have been assigned to other companies, in a feeble attempt to introduce a competitive train service market. These connections are mostly in other parts of the country. The entire privatization issue is not very fashionable at the moment.
If you visit Amsterdam, NS is normally the only train company you will deal with.
On the website you can find out the timetable for your trip, also in English.
TIP: Before any journey it is advisable to have a look on the NS website, to see if there is any service dirsuption or scheduled maintenance.
Amsterdam Sloterdijk train station
Train compartment in Dutch double decker train