The chef is responsible for everything about the food in a restaurant. He is responsible for buying the food, hiring and supervising the kitchen workers, creating the menus for the customers, and deciding the recipes (the details of how the cooking is done). Some chefs are trained and some are not trained.
The sous-chef (sometimes called an assistant chef) is the chef's main helper. A chef de partie is a 'line chef', in charge of a station in the kitchen. The commis chef works under a chef de partie. It is usually a training position.
The lines or stations in a big kitchen are: