Cooking sherry is a version of sherry which has been treated with salts and other additives to make it more shelf stable. Some cooks eschew cooking wines because of the added salt, which tends to flavor the final dish, sometimes unpleasantly. Cooking wines also tend to be inferior versions of their drinking cousins, sometimes lacking the flavor and complexity which they are supposed to add to a dish. Many stores stock cooking sherry along with other cooking wines, for cooks who want to use them.
Sherry is a type of fortified wine, meaning that a stronger liquor such as brandy is added to the wine. The elevated alcohol content of sherry makes it more shelf-stable, which was useful in sherry's early days since it meant that the wine could be shipped. The original sherry was from Jerez, in Spain, which came to be corrupted in English to Sherry. There are a range of sherry wines, from very dry fino to sweet cream sherry, and many dishes call for the addition of sherry to enhance the flavor.
True sherry can be stored in a cool dry place, unopened, for up to 15 years in some cases. Once opened, however, the sherry must be used within seven to 10 days, or the flavor will have faded and the wine will start to go sour. Sherry is traditionally opened for drinking before or after dinner, usually in small glasses since the wine has an intense, rich flavor.
The primary advantage of cooking sherry is that it can be kept after opening for quite some time. Since few recipes call for an entire bottle of sherry, cooks like to be able to open a bottle, use a small amount, and replace the bottle in the cupboard until it is needed again. However, the shelf stability of cooking sherry comes at a price. The salty sherry sometimes reacts unfavorably with the food, and it makes it difficult to control how salty dishes become.
Because of the added salt, this sherry is not suitable for drinking. While it is unlikely to make the consumer ill, it certainly will not taste terribly appealing. Some cooks prefer to use regular red or white wine as a replacement for cooking sherry, depending on the dish. The unused wine can be served with dinner. Other alcohol free substitutes such as vanilla, coffee, or soup stock may be used, depending on the recipe.