Sweet and sour sauce is a sauce which combines both sweet and sour flavors, creating a distinctively tangy mouthfeel. Many people associate this dish with Chinese cuisine, but in fact many cultures have a tradition of blending sweet and sour flavors in a variety of ways: cranberry sauce, for example, a New World food, is technically a form of sweet and sour sauce. There are a number of ways to prepare and use sweet and sour sauce, and it is widely used all over the world.
Sweetness and sourness are two of the primary and distinct tastes which can be detected by the tongue. People can also taste saltiness, bitterness, and a fifth taste known as “umami.” These five tastes combine in a wide variety of ways to create a huge library of flavors, from the bitter taste of coffee to the sweet taste of fresh fruits. Many cultures have a history of playing with contrasting tastes like sweet and sour, sweet and salty, or sweet and bitter in their cooking, using the interplay of flavors to enhance the flavor of their cuisines.
This sauce does indeed originate in China, with most evidence suggesting that it comes from Canton, the home of sweet and sour pork. Sweet and sour pork is, in fact, a common New Year's treat even today in this region of China, and it is likely that the sauce as well as the dish spread outwards from Canton. Hunan also lays a claim to the invention of sweet and sour sauce, and it is possible that it arose independently there, as the combination of sweet and sour is hardly rocket science to figure out.
In Chinese cooking, sweet and sour sauce is made with just two ingredients: sugar and vinegar. The sugar may be white or brown, with some cooks using molasses or other sweeteners like honey, and the vinegar may sometimes be replaced with rice wine. The distinctive red color which many Westerners associated with sweet and sour sauce is due to the addition of tomato ketchup, a Western invention.
In China, sweet and sour sauce can be used for dipping, and also as a cooking sauce. It may also be used to dress salads and other side dishes, depending on the region and the tastes of the cook. Outside of Asia, various regional takes on sweet and sour sauce are used in much the same way. For example, cooks in the American South may make ham with honey and orange juice, a combination of sweet and sour, and Italians are big fans of saba, a rich reduction of fresh grape juice which has both sweet and sour notes.