A double room is intended for two people. Instead of two single beds, it will have a larger double bed intended for two people, typically queen or king-sized. This type of room is best for couples (or people who don't mind sharing a bed).
How do hotels know if you have extra person?
Most hotels do notice when an additional guest arrives, either coming and going from the resort, or the room cleaning staff can notice. some hotels train their staff to notice. Of course, not every extra guest is noticed and some people do get away with it.