The eye is like a camera. It lets light in through the cornea, which is like a camera's aperture. The amount of light allowed in is controlled by the pupil, which opens and closes a bit like a shutter. The light focuses on the retina, which sends the image to the brain, acting as film would in order to record the light (the photo itself).
Other eye structures support the main activity of sight: Some carry fluids (such as tears and blood) to lubricate or nourish the eye. Others are muscles that allow the eye to move. Some protect the eye from injury (such as the lids and the epithelium of the cornea). And some are messengers, sending sensory information to the brain (such as the pain-sensing nerves in the cornea and the optic nerve behind the retina).