Single: A room assigned to one person. May have one or more beds.
Double: A room assigned to two people. May have one or more beds.
Twin: A room with two twin beds. May be occupied by one or more people.
So, it doesn't matter how many beds there are in either a single or double room, whereas there must be two "twin"-size beds in a twin room. A twin size bed usually has the measurement of 39" x 75", or 3 1/4" x 6 1/2".
Click the following links to see more pics, and you will see that a double room can denote a big bed for two people or two separate beds. It is designed for two people anyhow.