Light enters the eye through the cornea, pupil and lens. The light passes through the vitreous cavity - a space inside the eye, filled with a clear, jelly-like substance - and is focused on the retina.
The retina forms a picture from this focused light, and information about the picture is transmitted to the brain via the optic nerve. The retina has two parts. In the center is the macula, a small but very sensitive area which allows you to see tiny detail, thread a needle and recognize faces.
Surrounding the macula is the Peripheral retina. One hundred times less sensitive, but accounting for 95% of the retinal surface, it gives us side, or Peripheral, vision. When you see someone "out of the corner of your eye" it is the Peripheral retina picking up the picture. You may recognize a general shape in your Peripheral vision, but to see fine detail you must use your macula.
The entire eye is covered with an outer protective wall called the sclera.