There are people in Europe who do not know the difference between double room and twin room. Even the hoteliers sometimes do not know about the difference.
Double room is a room with a bed which two persons can sleep. On the other hand, twin room has two beds. Then, the beds are smaller than the bed placed in a double room, which one person can sleep on each bed. May be it is easier to say that there are two single beds.
Most European couples prefer to stay at double rooms, therefore European hotels have more double rooms than twin rooms. However, Japanese prefer to stay at twin rooms. Therefore hotels in Japan have more twin rooms that double rooms. Therefore if you want to stay in a double room during your stay in Japan, please be careful to choose the room.
Also the rate is different. Usually double room is slightly cheaper than twin room. Then, Japan does not have same rates for single person or two persons occupancy. Which means that the double room for two persons is always expensive that a solo use.
There are also a room called semi-double room. There is one bed in the room but the bed is wider than the single bed but narrower than the double bed. The room can accommodate two persons and of course cheaper than double room. Then, some hotels offer semi-double rooms but not offer any double room. They can have just single, semi-double and twin rooms.