In twin room, the beds are separate where as in double there is no separation of the bed. Couples usually prefer double rooms. On the other hand, two girls or two guys traveling would go for twin room rather then double room.
1 Twin bed
A single room is for one person and contains a single bed, and will usually be quite small (but that's the norm for European hotel rooms anyway). A double room is for two people and may contain a double bed or two singles. Sometimes, when you book a single room, the hotel may put you…
1 Single bed