Double room: double rooms are assigned to two people; expect one double bed, or two twin beds depending on the hotel. Triple room: as the name might suggest, this room is equipped for three people to stay. The room will have a combination of either three twin beds, one double bed and a twin, or two double beds.
double room (plural double rooms) A room, especially in a hotel, that is designed for two people and has two single beds (sometimes called a twin room) or one double bed.
A double room is a room intended for two people, usually a couple, to stay in. One person occupying a double room has to pay a supplement. She needs two double rooms for four guests.
A single room is for one person, a double room is for two people, a twin room is for two people who will have 1 small individual bed for each, and a family room is for four or more people.
Most hotels price their rooms as doubles or family rooms and don't discount them if they are only occupied by one person. This is justified by the hotel because they lose money with one occupant as the cost of provisions and cleaning is still the same, regardless of how many people stay in the room.
A normal double room will not have enough spare space to add a third bed. An adult child sleeping with their parents would be regarded as unusual. Family rooms are child sized beds.
A twin room will have two separate beds, but usually these can be coupled to make a double. As for doubles being smaller than in America, Europeans are smaller and prefer closeness to each other.
The Double is also called a Full and is larger than a Twin but smaller than a Queen. We usually book rooms with Queen size beds but the website makes it sound as though the Doubles are larger. The rooms are larger and the beds appear larger in the photo.