These are all different states or phases in which a fluid can exist. H20 exists as a solid (ice), liquid (water), vapour (saturated steam), and a gas (superheated steam) – it depends on the temperature and pressure which phase is current. Practically the most significant difference between liquids and vapours/gases is the compressibility. Liquids are for most practical purposes incompressible where as the density of gas and vapours varies with pressure.