Just to clarify..... the article that Bill pointed out is right. You do not want TOO much space behind the fridge. Too much space and the air will not draft effeciently to the roof vent. AND if you have a fan it should be mounted high, a close as possible to the roof vent, to create draft.
Your fireplace works the same way in your house or heating chimney. The idea is to create draft, not push out the hot air.