Ice is the solid form of water. This happens when water is cooled down to 0'C, or 273K at standard atmospheric pressure. Ice can be formed at higher temperatures in pressurized environments, and water will remain a liquid or gas until -30C at lower pressures. Ice formed at high pressure has a different crystal structure and density than ordinary ice.
An unusual feature of ice freezed at atmospheric pressure is that the solid form is less dense than the liquid form of water. This is due to hydrogen bonds between the water molecules which line up molecules less efficiently (in terms of space) when water is frozen.