Fish sauce is much used in South-east Asian cooking to add saltiness to the dishes. Asian fish sauce is made from anchovies, salt and water, and it needs to be used in moderation because it is intensely flavoured. The variety from Vietnam is generally called nước mắm, and the similar condiment from Thailand is called nam pla. In China, it is called yu lu (魚露).
A similar fish sauce was frequently used in ancient Roman cooking, where in Latin it is known as garum. In English it was formerly known as fishpickle. The original Worcestershire sauce was a similar product, brought to England from India.
Here is a recipe from the 1881 Household Cyclopedia for an antique fish sauce.
Take 1 pound of anchovies, a quart of claret (a dry red wine), of a pint of white wine vinegar, 1/2 an ounce of cloves and mace, 2 rages of ginger sliced, a little black pepper, the peel of a lemon, a piece of horseradish, a large onion, a bunch of thyme and savory; set all these over a slow fire to simmer an hour, then strain it through a sieve; when cold put it in a bottle with the spice, but not the herbs. To a large coffeecupful cold, put a pound of butter; stir it over the fire till it is as thick as cream; shake the bottle when used, and put no water to the butter.
see Wikipedia Cookbook
