Soy sauce is traditionally used a lot in Asian cuisine, both in cooking and as a condiment. It is produced by fermenting soy beans, often combined with a grain, with Aspergillus molds, water and salt. The fermentation yields a paste that is pressed to separate the liquid, which is the soy sauce, and solid residues that are used as animal feed. Today, soy sauce can also be produced chemically instead of by fermentation, which is much faster (3 days instead of 3 months), but has a different taste than the original soy sauce. There are many different varieties with their own flavour, texture and cooking properties, but all are salty, brownish liquids with umami taste, and work as a flavour enhancer. Originally making soy sauce was probably a way to stretch costly salt.
The three most commonly used soy sauces in the Western world are Chinese, Indonesian and Japanese, but almost all Asian countries have their own variety.
All Chinese soy sauces are made primarily with soy beans, with low amounts of other grains, and can roughly be divided in two categories, brewed and blended. Brewed soy sauce has no additives, it is only fermented, while blended is aged more and has additives to enhance the sweet and umami flavours. Light soy sauce (which can be seen on the photo below) is the most common example of the first category. It is quite thin, salty, has a light colouring and is used for seasoning and dipping. This is also the soy sauce I most use in cooking, for example for egg fried rice. Dark soy sauce is the most common example of the second category, which contains caramel and molasses to give it its dark, rich flavour and texture together with the prolonged ageing. It is less salty than light soy sauce and it is mainly used during cooking, because the flavour develops during heating. For me, this soy sauce is too distinct in flavour, it is difficult to not let it overpower the dish.
Because of the colonial history of the Netherlands in Indonesia (back then Dutch West Indies), ketjap (also kecap) is most often used as soy sauce in the Netherlands, but in the rest of the world it is less well known. There are two main varieties, ketjap manis and ketjap asin. Ketjap manis (which can be seen on the photo below) is very thick, sweet and not so salty, because of the addition of palm sugar. I usually add it last-minute to a dish, because the flavour changes unpleasantly after too much heating, and it is also prone to burning. This is my preferred soy sauce for making nasi goreng. Ketjap asin is a salty soy sauce similar to Chinese light soy sauce, but slightly thicker and has a stronger flavour.
Japanese soy sauces are generally made with quite high amounts of grains and are very salty. As you can see on the photo below, I have a bottle of reduced salt Japanese soy, because the regular I think is much too salty. There are five very different variants, of which koikuchi is the most common.
So, next time when a recipe asks for soy sauce, think about which kind it should be. Generally you are safest by using Chinese light soy, because of its quite neutral flavours. But when cooking Indonesian or Japanese dishes, your dishes will greatly improve by using the appropriate soy sauce.
In the Netherlands ordinary Japanese and ketjap can be bought at the supermarket. Although I think that some supermarkets have reduced salt Japanese and Chinese soy sauce in their assortment, not all have it. In that case, check your local toko or Asian supermarket.