The retina lines the inside of the back of the eye. It is composed of the macula and the peripheral retina. The retina can be compared to the film in a camera (or the light sensor in a digital camera). Light coming into the eye passes through the cornea and lens (function like a camera lens) and focuses on the retina (which functions like the film or image sensor). The image on the retina is upside down, just like a camera, but when the brain processes the image it turns right side up.
Retinal problems usually cause painless disturbances of the vision. See the links to the left margin of this page for topics on common retinal problems.