A Fish Sauce is generally defined as a sauce made with fish as a main ingredient in the sauce, rather than as a sauce to be served with fish.
Though versions have been made since Ancient Greece, most people now think of Fish Sauce as a fermented sauce made from fish throughout Asia, with regional variations of course. The most well-known style in the West currently (2007) is Thai.
Fish Sauce in general is made from seafood such as shrimp or shellfish, and either fish so small they don’t otherwise have a commercial value, or parts of fish with no commercial value. The fish used can be freshwater or saltwater, raw or dried, and one type of fish, or several. The whole fish may be used, or only the head, innards and blood.
The fish are fermented. A short fermentation preserves a fishier taste; a longer one changes the taste far more.
All Fish Sauces have a strong, salty taste. They are used in place of both salt and soy sauce.
Some versions add herbs and spices.
The Greeks and Romans made fish sauces that were sold commercially for hundreds of years, though their exact recipes have been lost.
Nowadays, the word “garum” tends to be used generically as the Roman word for “fish sauce”, but that wasn’t actually the case. “Garum” was a Roman fish sauce made from the entrails and blood of a mixture of fish, but predominantly mackerel. “Liquamen” was a fish sauce made from anchovy.
Nguyen, Mary. Vietnamese Fish Sauce. Trade Environment Database (TED) Case Studies, Number 769. 2004. Retrieved July 2006 from http://www.american.edu/TED/vietnam-fish-sauce.htm.