[tim333]

Layman's - stuff like water it made of molecules, H2O in this case and the H has a positive charge and the O a negative charge (the electrons head over to the O atom due to quantum effects). These charges tend to make the molecules attract and stick together (the -ve are attracted to the + charges). The temperature of anything corresponds roughly to the kinetic energy of the bits of stuff it's made of. At 0K everything is still. At 373K (== 100C) the water molecules have enough energy at normal pressure for the molecules to fly apart and have your kettle boil. You can raise this temperature by putting the water in a pressure cooker to physically push the molecules back together - this works to about 120C in a normal pressure cooker. At some temperature however, no matter how much pressure you put on, the molecules do not stick to each other to form a liquid because they have too much kinetic energy and keep flying apart. This happens at around 647K in water. Above that the steam/liquid is called supercritical. This is important in power generation because you can squish it as hard as you like without worrying about it condensing.