Why would plugging in an AC unit in a wall receptacle cause the fuse to blow or breaker to trip?

Is the receptacle a 110 or a 220 outlet? If it's a 110, it needs to be a 220. Are there other appliances, lights, etc. wired on the same outlet? If so, you may have to re-wire so that no other appliances, lights, etc. are wired into the same breaker. Usually the larger appliances such as air conditioners, central heat systems are wired to a separate breaker or fuse.