Simply put, a blast freezer is just a freezer which is very cold. Blast freezers are sometimes referred to as shock freezers. The idea of this type of freezer is to very rapidly bring down the temperature of (usually) foodstuffs or fresh produce, freezing them very quickly.
They are widely used in the frozen food industry for things like iced cream, pre-prepared meals and vegetables or fish. A lot of commercial kitchens also use them for some things.
When you freeze food (or anything else for that matter) the water inside crystallises into ice. The longer the freezing process takes, the larger the ice crystals.
Larger ice crystals damage materials by causing phenomena like cell bursting, which affects quality and flavour of foods.Very rapid freezing within a blast freezer causes the crystals formed to be very small, which does less damage and preserves food at a higher quality.
Even more usefully, once the food is "blast frozen" it can be moved into a normal freezer for longer term storage - as long as this one stays cold enough for the food to stay frozen, of course.
A lot of large blast freezers use blower fans which force cold air across the contents of the freezer in order to rapidly chill it. Others can work in other ways depending on the design.
Blast freezers can be equipped with trays which move about in order to position foods, and often include various compartments for freezing. An average freezer can normally handle foods at room temperature or even heated as long as the capacity of the freezer is not overloaded.
Put a lot of hot food in the freezer and its temperature will rise which will negatively affect its cooling capacity. Some specialist companies offer portable blast freezers which can be moved outside to help with temperature control and ventilation.
Aside from the integrity, quality and taste of the food, blast freezing also offers a health and safety benefit in that extreme cold slows down the growth of bacteria.
Provided that the food is handled safely and then put into the freezer immediately after preparation, there is a greatly reduced risk of contamination.
This benefit to food safety is one of the reasons why blast freezing is very popular in commercial food preparation areas, as it greatly reduces the risk to customers and consumers.