What is fish sauce?
Fish sauce is basically made from fermented fish and or shellfish. It can vary from region to region but is primarily Asian in usage.
" it is that salty, smelly brown liquid made from fish that is the single, most important flavoring ingredient in Thai cooking (also well-loved in Laos, Cambodia, Vietnam, Burma and the Philippines). Used like salt in western cooking and soy sauce in Chinese cooking, good-quality fish sauce imparts a distinct aroma and flavor all its own. It is indispensable in the Thai kitchen as Thai food wouldn't be quite the same without it.
Called "nam bplah" in Thai, or literally "fish water," genuine fish sauce is the water, or juice, in the flesh of fish that is extracted in the process of prolonged salting and fermentation. It is made from small fish that would otherwise have little value for consumption. This can either be freshwater or saltwater fish, though today, most fish sauce is made from the latter as pollution and dams have drastically reduced the once plentiful supply of freshwater fish in the heartlands of Southeast Asia."
All of the above, plus it stinks like crazy but adds a taste that will addict you to it. Being fermented it will last for a few years in the fridge.