The satin weave is characterized by four or more cool fill or weft yarns floating over a warp yarn or vice versa, four warp yarns floating over a single weft yarn. This explains the 'floatiness' of the appearance.
The fabric is a satin if the fabric is warp faced (the fabric predominantly shows warp threads). The fabric is a sateen if it is weft faced meaning only the weft threads show. The float is in the warp or vertical direction for satin and with sateen the float is in the horizontal or filling direction.