Fish sauce, or "nam pla" in Thai, is one of the basic ingredients in Thai cooking. It has a rich, translucent reddish-golden brown color and is added liberally to nearly all Thai dishes. It is often used as a marinade for fish and meat, as well as a condiment (usually mixed with fresh-cut chilies and lime juice). If you travel to Thailand, you may come across this "sauce" on tables in Thai restaurants; Thais add a little fish sauce to their meals the same way we would use salt and pepper.
What Fish Sauce Is
Good fish sauces are made from a mixture of fish and salt that has been allowed to ferment for 1 year to 18 months. Anchovies are typically used, although some fish sauces are also made from other types of fish or squid. The basic ingredients of a good fish sauce are fish, water, and salt. Sugar may also be added but isn't necessary.
Where to Buy Fish Sauce
These days, most supermarkets sell fish sauce (look for it in the Asian section), but you may find the best brands, like Squid Brand Fish Sauce, sold in Asian food stores. You'll find a good selection of fish sauces at nearly any Chinese/Vietnamese/Thai food market. Look for tall bottles with "Fish Sauce" and the ingredients displayed on the label (just fish extract, salt, and water—other ingredients aren't necessary). It should be made in Thailand or Vietnam. You can also order nam pla online.
A Vegetarian Substitution
Although it is a misnomer, vegetarian (and vegan) fish sauce does exist and is often made of seaweed instead of fish. You may not find it in a Thai food store, but nearly all the Vietnamese food markets carry it. However, better substitutes are Golden Mountain sauce, just plain soy sauce, or a combination of the two. To purchase Golden Mountain Sauce you will need to shop at a good Asian market.
Fish Sauce and Sodium
For those who are concerned about their sodium intake, using fish sauce can be a bit of a dilemma. Not to worry. While the sodium content of fish sauce seems outrageous when you look at the serving size on the label, remember this amount will be distributed throughout the dish you're cooking (for example, a Thai curry), so you won't be consuming all of it—at least not in one sitting. If you would like to cut back, add only a portion of the fish sauce called for in the recipe, then finish off the remainder of the measurement with sea salt. Sea salt has only a fraction of the sodium found in regular table salt and is much better for you.
Avoid the Fishy Smell
Not to state the obvious, but fish sauce smells fishy. The secret to counteracting this odor is to add lime juice (and in most Thai recipes you will find fish sauce paired with lime juice). Once you are used to Southeast Asian cooking, the smell of fish sauce will become a normal part of your culinary sensory world. Until then, if you find either the dish you're cooking or your hands smell fishy, try squeezing over some fresh lime juice and see if it helps.