Chinese dumpling (餃子). A mixture of minced pork, cabbage and Chinese chives (nira) is wrapped into a thinly rolled dough skin, which is then sealed by pressing the edges together, and steam-fried. Sesame oil, garlic and ginger may be added to the mixture.
Originally a Chinese dish (jiaozi), as so many things gyoza have been thoroughly Japanified as vital and indispensable items in the Chuka cuisine. Japanese gyoza have thinner wrappers, are lightly flavored with soy but also more garlicky than the Chinese version.
Gyoza can be found anywhere in Japan, from Chuka restaurants or ramen shops where they serve as a side dish, to supermarkets and street stalls. Gyoza are good with beer and therefore often served in izakaya. Cities with Chinese towns as Kobe and Yokohama have good gyoza and also Utsunomiya has made its gyoza restaurants one of its sales points.
The most popular way of preparation is Yaki-gyoza (焼き餃子), where the dumpling first is pan-fried on the bottom and then steamed on top in a broth. Gyoza therefore are crisp and browned on the bottom, sticking lightly to the pan, and soft on the top.
Other methods of preparing gyoza whicch are less popular in Japan include boiled Sui-gyoza (水餃子) and deep-fried Age-gyoza (揚げ餃子).
Supermarkets also sell frozen gyoza. Grocery stores provide the dough pieces for those who want to experiment with own cooking. It is nice to be creative with the fillings.
In English gyoza are known as "Peking ravioli" or "potstickers."
Gyoza post on Just Hungry.
[Written with information from Wikipedia]