Building codes regulate the amount of fresh air that every room in your house must receive. Some rooms, such as living rooms and sleeping areas, must have natural ventilation from windows; bathrooms, laundry rooms, and kitchens can be mechanically ventilated.
Mechanical ventilation requires ducts that go from the room to the outside of the house; to circulate the necessary air there must also be a fan, which can be turned on separately or goes on automatically whenever the lights in the room are turned on.
Or, there is a central fan on the roof controlled by a timer that switches it on periodically and allows it to run long enough to change the air in the rooms that are ducted to it.
The system you use depends on your local building code, which, which, you will find, is very specific about the ducts, fan sizes, how they are installed, and the size of the rooms to be vented.
Mechanical ventilation can be a renovators savior, particularly if you are dealing with a deep, narrow, urban row house with windows only in it’s front and back walls. Given a mechanical ventilation system, you can place the bathrooms, kitchens, laundry rooms in the middle of such a house and use the windowed ends for the living and sleeping rooms.
But you must know exactly what your building code demands before you start building interior rooms, because the duct work will have to be hidden in some manner — presumably in the ceiling or up a wall somewhere; how you hide those ducts will have a lot to do with how you design the rooms.