Freezing of water occurs when the kinetic energy of the water molecules is reduced to the point that they can no longer overcome the attractive forces between the molecules. This causes them to become arranged into a more structured form, forming ice crystals. As the temperature decreases further, the water molecules will become more organized and eventually form a solid. This is why water freezes at 0 degrees Celsius (32 degrees Fahrenheit).
The density and volume of water in its solid state (i.e. ice) is lower than in its liquid state. This is because the molecules of ice are more spread out than those of liquid water, which makes the solid state less dense and occupies more space.