I've made a booking for a weekend in Holland online via ebookers. When I completed the transaction and looked at the final receipt, it said "prepayment required" ... does that mean I need to pay at the desk at the end of my stay or does it mean I've paid already when I made the booking online and won't be charged when I leave ?
For all products apart from certain hotels full payment is required at the time of booking in order secure your reservation.
For hotel rates that do not require prepayment, your credit or debit card typically would be charged at check-out.
From the above, it clearly means that you have fully paid for the hotel already, and you will not have to pay anything at checkout time, unless you have ordered room service or anything else that requires payment.
The term Prepayment covers any payment made prior to arriving at the hotel. From ebookers.com terms it seems that their prepayment is done at the time of booking. Others such as Agoda have a variable policies as to when they will bill your credit card, sometimes at booking, sometimes at a set date prior to your arrival (like a week before or two weeks before, etc).
Paying when you check into the hotel would not be considered prepayment.