Why is the reason the motion picture camera was invented?

I under stand that motion picture cameras have adjustable shutters. In turn you can then adjust the aprature to keep the same exposure, just as you would with a still camera. Can you properly explain this for a motion picture camera? The reason I'm asking this question, is because they're filming a movie in my home town, and everybody never sees the camera brought in or out of the building. I told them the camera is put together. I'd like to know how a movie camera works. I understand there is a circular rotating shutter, and the film never stops. Please explain this. Thanks