A single room is a room with enough space for a single bed, with a single bed. However, a double room does not necessarily mean there is a double bed in the room, although it does mean it is big enough for one. You might want to clarify with an advertiser whether a double room actually has a double bed in..
Some standard double rooms are meant for a couple; these have just one queen- or king-sized bed. Other standard double rooms have two double or queen-sized beds placed side by side a few feet apart. Sometimes families of four with young children who can share a bed opt for the two beds.
A single or double room is typically used for 1-2 persons travelling as the bedding configuration is normally one bed. A twin room is for 2 persons travelling however the room normally has 2 beds.