Rochelle Frank is absolutely correct; the Romans were the ones who invented fish sauce but South East Asia is where most of it comes from now. My wife is from Cambodia and she uses it to cook all the time. Here is a list of ingredients:
Anchovy Extract, water, salt, fructose, and hydrolyzed vegetable protein.
1 Tbsp. Contains approximately 1800 mg or 75% Sodium, 2g or 1% total carbohydrate, 2g of sugar, and 2g of protein.
This is one particular brand and I have seen other brands with slightly different numbers and sometime use different kinds of fish. I hope that this answers your question!