It depends on what type of room rate you booked. Essentially there are two types.

Pre-Paid and Flex.

Pre-Paid also known as advanced purchase, normally is cheaper, but you pay for the room in advance. Sometimes your card will get charged within 24 hours of making the reservations other times it might not happen until a few days before check-in, depending on how efficient the hotel is they may not even charge your card until the day of check-in. Once you have booked the room, the T&C's say that if it is cancel or change dates you have to pay the full amount. There are ways around this search the forum. When you check-in they will swipe your card for any charges made against your room. Then when you check-out they will charge your card these sundries.

The Flex rate is more expensive, but you can normally cancel up to 4 or 6pm on the day, l think each hotel is different. When you check-in they will swipe your card and take a holding deposit for any charges to your room. At the end of the stay they will then charge your card for the accommodation and then any other expenses.

Hope this helps.