Stir frying is a Chinese cooking technique commonly used in Chinese restaurants because of its fast cooking speed. Cantonese restaurant patrons judge the chefs by their "wok qi" (their ability to bring out the qi of the wok, which shows in the food as the look, smell and taste).
A round bottom pan, called a wok is heated to very high temperature, some oil is put in, followed by seasoning and the food items. The food is stirred and tossed very quickly using a big metal spatula. Some chefs would lift the wok to the side to let the flame light the oil or a dash of wine spirit on the food to give it extra flavor. Most dishes are cooked this way within 30 seconds.
Some dishes that required longer cooking are done by adding a few dashes of water after the stirring, then the wok is covered with a lid. As soon as the steam start to come out under the lid, the dish is ready. Basically, in this case, the food is stir fried in high heat for the flavor and then steamed to make sure it is fully cooked.
Stir frying at home often cannot achieve the same flavor as in restaurants mainly because the wok is not hot enough and the wok is too small to allow fast tossing. Anyway, most home kitchens are not equipped to handle the large amount of oil vapour as by product of proper stir frying.
Many western cooks on TV demostrate stir frying on low heat with a tiny wok and a stirring motion comparable to tossing a salad. That is western stir frying, not the way a Chinese chef would do it.