WATER MOLECULE H2O: Water consists of one oxygen atom covalently joined to two hydrogen atoms. H2O is a highly polarised molecule which leads to hydrogen bonding among the molecules. In the image above, the central oxygen atom (red) is pictured as a sphere drawn at the scale of its covalent radius just touching two hydrogen atoms (green) drawn at the scale of their covalent radii. The transparent spheres represent the "surfaces" of each molecule and are created from the van der Waals radii of the constituent atoms.
STEAM: water boils at 100oC. The water molecules rapidly gain energy and the more energetic ones leave the liquid phase and enter the gas phase. This is what might happen above a saucepan of boiling water, for example. As steam is produced, it exerts a gas pressure and this is what is harnessed to drive steam engines and steam turbines. Much of the industrial revolution was powered by steam in factories, trains and steamships. Steam power is also used in nuclear energy generation. The nuclear reaction generates huge amounts of heat and this is used to boil water. The resulting steam drives turbines to produce electricity. Steam is used domestically for steam cleaning, ironing and cooking.