Simply put, retinoblastoma is a rare malignant tumor of the retina that most commonly affects young children.  It is a cancer of the eye that starts in the retina, and although considered an early childhood cancer, it can occur (rarely) in adults as well.  This type of eye cancer effects the retina, as mentioned, which is the part of the eye that is responsible for sensing light and sending images of what you see to the brain.  The layer of nerve tissue that makes up the retina sends signals of the light it senses through the optic nerve to the brain.  It is in the brain that these signals are translated into the images we ultimately see.