Usually done with shrimp or anchovies. Traditional preparation is all of taking a pile of them, salting them, and letting them sit in the sun for a while, until it's thick with flies. It's said modern production is a bit better.

A fish sauce is also done in a similar manner, with the seafood sitting in a barrel, and then after it's set for a while, it's squished with a board and the juice that runs out is kept. Add a couple ingredients and get Worcestershire Sauce.