In the traditional French kitchen layering system called the Kitchen Brigade, the chef de cuisine and the executive sub-chef depend on the executive chef. If both roles are present, the chef de cuisine is second in command, while the subchef is next. In managerial chef positions, there is an established hierarchy because these chefs have the most responsibility in the kitchen to ensure the overall success of the restaurant. The job of the junior chef is to work with the most experienced chefs to gain the experience and skills needed to become seasonal chefs themselves.
As an apprentice chef who has attended a university course or hospitality school, you'll be ahead of another chef who hasn't. Depending on the restaurant and on the person himself, like the executive directors of the business world, the head chef usually leaves much of the daily operation of the kitchen to people at a lower level, such as the subchef. Becoming a chef requires years of education and experience, from entry-level positions to the ultimate goal of executive chef. Some establishments will combine the role of executive chef and head chef if it is not necessary to have both.
Chef de Cuisine is the traditional French term and, although it's a little more common in European kitchens, head chef is the most frequently used title around the world. The difference between a chef and a cook is that a chef assumes a more managerial role with his responsibilities.