What is fish sauce?
Fish sauce is a pungent liquid flavoring made from salted, fermented fish. A staple of Southeast Asian cuisines, most varieties are made with anchovies, salt and sugar.
You can usually find fish sauce on the international aisle at the grocery store. If not, you'll certainly find several varieties at any Asian market. (Chinese fish sauce may be labeled fish gravy.) Store fish sauce in the pantry, not in the refrigerator -- the cold air will cause salt crystals to form.
In a pinch, or if you want to make a dish vegetarian, you can substitute soy sauce for fish sauce. Substituting fish sauce for soy sauce is a riskier move. Plenty of people do it, but if you're a casual user of or newcomer to fish sauce, proceed with caution: it's potent.
It takes 10-15 pounds of fish to make 1 liter of fish sauce, so it's not surprising that the dark liquid smells intensely fishy. The flavor, however, is not so much fishy as it is salty, tangy and vinegary. Paired with lime juice and broth, fish sauce creates a bold base for this soups.
Recipe: Broiled Thai Shrimp (Coothink)
Recipe: Beef Pho (Cookthink)
Recipe: Thai Shrimp, Coconut And Mushroom Soup (Cookthink)
Reference: Why is it called Worcestershire sauce? (Cookthink)