The difference is purely one of temperature. Water vapour is the same temperature as the air in which it is spread. Steam is above the local boiling-point of water (usually taken to be 100°C, but varies with local weather and altitude). It is also a general assumption that "a cloud of steam" will be largely steam / water vapour, whereas "water vapour" is typically only a few percent of the volume of the air. Chemically, steam and water vapour are identical.