The human eye is much like a camera - or rather, cameras were designed to be much like the human eye. In both cases, light is focused onto a certain target; in the case of a standard camera, that target is a piece of film. In the eye, light is focused on the retina. The natural crystalline lens and the cornea serve as the lenses that focus the light onto the retina. The iris is the colored part of the eye and surrounds the pupil, which enlarges and contracts to regulate the amount of light entering the eye.
Once the light has reached the retina, the optic nerve carries the light to the brain, where it is translated into the images you see, much like having a roll of film developed. And just like poorly-focused photographs will result in blurry prints, your vision will be blurred if any part of your focusing mechanisms are not functioning properly. The most common cause of refractive error is a cornea that is too steep or too flat, causing the light to focus at the wrong distance.
The remaining area inside the eye is filled with a clear gelatinous substance known as the vitreous, while the sclera, or the white of the eye, serves as the protective outer shell that could be equated to the plastic body of a camera.