As above. "Double" would usually mean one double bed; you might be offered a "king-size double" and so on, usually for a slightly higher cost. Sometimes, though, these rooms with larger double beds are given less transparent names ("premium", "executive", and so on).
"Twin" means two separate beds, obviously; in my experience this would more commonly be two single beds, but I have known a double and a single bed, and even two double beds, to be called a "twin".