Contract type is usually settled before the first working day, often in a language the new hire is still learning, and it determines whether a job can simply end on a date or requires a formal dismissal route. The difference surfaces later, when a renewal conversation either happens or does not.
Bepaalde tijd is a fixed-term contract that simply ends on its agreed date — no dismissal procedure needed, it just runs out. The one obligation on the employer: for contracts of six months or longer, they must give aanzegging, written notice at least a month before the end date saying whether it will be renewed and on what terms; skip that and they owe you compensation regardless of what happens next. Onbepaalde tijd is an indefinite, permanent contract with no end date, and ending it requires a valid legal ground — through the UWV (employee insurance agency), the subdistrict court, or a mutually agreed settlement — it can't just be allowed to lapse. The practical difference for you: a fixed-term contract not renewing isn't a dismissal in the legal sense, even though it feels like losing your job.