South-east Asian fish sauces are generally made from fermented anchovies, lots of salt and water. They have a dark amber colour and exhibit a mild fish smell and a sharp almost metallic taste of fish and salt.
This sauce is so versatile and can be used to enhance the flavours of many dishes due to its saltiness and also to adjust the balance of sour and sweet flavours. I find the sauce amazing and always sample some when cooking with it :).
Just like anything, fish sauces run the gamut from low to high quality so it pays to do a bit of research and of course you get what you pay for.
Here is the brand I use from Thailand (squid brand):
The use of fermented fish sauces in European cooking goes back to ancient greece and the Roman Empire, presumably when some bright spark tried some sludgy fish that had been sitting in the sun for a few days. I don’t recommend trying that :).