There are a number of different kinds of slush drinks:
Frozen uncarbonated beverages are made by freezing a non-carbonated juice or other liquid. Machines for producing these do not require a pressure chamber, and as a result, they are much cheaper and easier to maintain. There are variations including frozen carbonated beverages, typified by the Slurpee or ICEE, which are made by freezing a carbonated drink. Machines for producing these are complicated and expensive, and notably require a carbon dioxide supply. They make a very fine and 'dry' slush. There are also frozen coffee beverages and frozen alcoholic beverages, as well as more conventional slush drinks. There are also "instant" slush drinks formed via supercooling. Several examples include Slushie, which allows consumers to make beverages that turn to slush upon opening using a conventional freezer; the Chill Chamber, which allows businesses to store beverages at below freezing temperatures that turn to slush upon opening; and supercooled Sprite from Coca-Cola (briefly marketed in the UK), which required special vending machines to store the bottles in a supercooled state so that they would turn to slush upon opening.
Chipped ice in water is the standard for the freezing point of water, 0 °C (32 °F). Ice made from water can be 0 °C, or a much lower temperature. The agitation of the machines is partially to keep the water from freezing solid. Some of the drinks have additives to make the freezing temperature of the mix lower, so that the drink can be much colder than a water slush drink.
Slush is made by a mixture of sugar and water. To prevent the mixture from freezing solid, there must be between 12% - 22% of sugar present in the solution. The sugar acts as an antifreeze in the solution. The slush machine stirs or rotates the mixture at a constant speed so that the sugar and water molecules bond together before the water gets a chance to freeze. In this way, a soft, wet slurry mixture is formed.