Soy sauce is one of the most commonly used seasonings in Chinese cuisine. It contains just a few ingredients: soy beans and wheat are fermented, then mixed with salt and water. Depending on how long it's aged, this basic mixture can be made into light, regular (what most people are referring to when they say soy sauce without any qualifying adjective) and dark. Other types include low-sodium and mushroom soy sauce.
Soy sauce is made by crushing soy beans and wheat, steaming them, then mixing in a mould to start the fermentation process. After adding the salt and water, the mixture is aged and fermented in stainless steel or plastic vats, or in wooden casks. It is strained, pasturised and then bottled.
While most people would describe soy sauce as salty, the flavour is much more complex, varying from brand to brand. It can also taste sweet, caramelised, or winey (from the fermentation). The soy sauce aged in traditional wooden casks has subtle wood flavours. Lesser brands (not necessarily the cheaper ones) taste harsh and metallic.
The two types used most often in Chinese cuisines are regular and light. Although it looks more concentrated, regular soy is actually less salty than light. In a typical Chinese marinade, regular is used with salt (along with other flavourings such as rice wine and white pepper) because it is not salty enough on its own, and simply adding more soy to give sufficient saltiness would overwhelm the dish.
Regular soy is used to give richness and depth to dishes where the dark colour does not detract. It's also the variety used as a dipping sauce. Light soy is thinner, paler and saltier than regular. It is typically used in seafood dishes where you would not want the brown colour that comes from regular soy. Dark soy (sometimes called black or thick soy) is a rich, salty-sweet syrupy mixture which is used frequently in Malaysian, Singaporean and Indonesian cuisines. It gives flavour and colour to such noodle dishes as mee goreng and char kway teow, and is sometimes served as one of the dipping sauces for Hainan chicken.
Although soy sauce is also used in Korean and Japanese cuisines, you do not have to limit it to Asian food. It can give a subtle flavour that people cannot always identify. For a delicious roast pork, rub soy sauce and black pepper into the meat - the soy gives the roast a lovely, deep colour. It is also good as a marinade (along with rice wine, garlic and ginger) for barbecued steaks. If you like mayonnaise-based dipping sauces for blanched vegetables, try mixing a little soy sauce and sesame oil into Hellman's mayonnaise. This is especially good for broccoli and string beans.