The casing is to protect the inside of the camera from being damaged. It is a case and protection. If there were not a case things could easily be damaged.
Shutter: The shutter is a sealed box that does not allow light into the camera.
Lens: The lens redirect the beams of light bouncing off the object to form an image that looks like the scene.
Film: The film is light sensitive emulision on a plastic base.
Flash circuit: The flash circuit makes a short flash of bright light when the shutter is released.
Gas Discharge Tube: A tube filled with gas and X enon