I have noticed there are always these two roles mentioned in the final credits of every movie. What do these people do?
In North American filmmaking, the key grip supervises all grip (lighting and rigging) crews and reports to the director of photography.
Before filming begins, the key grip attends location scouts and meets the director of photography to determine what additional tools (location-specific motor vehicles, dollies, cranes, mounts, etc.) will be needed, orders and prepares required equipment, and transports equipment to the filming location. Once on set, the key grip coordinates with the electric and camera departments alongside the director of photography to control lighting and camera movement and setup. As a supervisor, the key grip directs the crew of grips, many with specialized skills such as dolly grips, crane operators, or special equipment operators.
In a film crew there are two kinds of best boy: best boy electric and best boy grip. They are assistants to their department heads, the gaffer (in charge of electricals) and the key grip (lighting and rigging), respectively. In short, the best boy acts as the foreman for the department. A woman who performs the duties of a best boy may be called best girl. All are sometimes called best person.
Best boys are responsible for the day-to-day operation of the lighting or grip department. Their many responsibilities include the hiring, scheduling, and management of crew; the renting, ordering, inventory, and returning of equipment; workplace safety and maintaining discipline within their department; completing timecards and other paperwork; stocking of expendables; loading and unloading production trucks; planning and implementing the lighting or rigging of locations and/or sound stages; coordinating with rigging crews and additional photography units (if applicable); handling relations with the other production departments; overseeing the application of union rules (where relevant); and serving as the day-to-day representative of the department with the unit production manager and coordinator of their department.