Trailers is a term still used in the UK. I would expect a preview to be an advance showing such as you sometimes see in the theatre; that is, a full performance made available to a limited or prebooked audience before the official run starts.

There are proper advertisements between showings as well of course, for insurance, car hire or Fred's takeway pizzas and so on. I think we generally use the abbreviation 'adverts' rather than 'ads' although that term is used for 'small ads' in newspapers.