The retina is the light sensitive area at the back of the eye socket. It contains cells that react to light and pass those signals on as nerve impulses to the brain.
In humans, there are two types of light sensitive cells in the retina; rods, that detect low levels of light, and cones, that can detect the color of light. Rods are so sensitive to light that they can detect a single photon.
The blood vessels and nerves that lead into the retina actually pass through it. There are no rods or cones in the area where the blood vessels and nerve pass through, which is referred to as the blind spot.
The retina's shape is curved only by the outward pressure of the fluid in the eyeball - it is not attached to the skull. As such, a "detached retina", where the retina comes away from the eye socket, is somewhat of a misnomer as what happens is that the eyeball leaks fluid which deforms the retina's shape.
The retina can be viewed by a physician through the iris by using a bright light and a scope. In this way, the physician can determine if there are any clots or other malformities of the retina if the patient complaints of poor vision.