Retinal detachment occurs when the inner layer of the eyeball, the retina, becomes separated from the nerve tissues and blood supply underneath it.
The condition is painless, but if not treated promptly it can irreparably damage vision. Once a detachment has occurred, symptoms include a clouding effect that has been likened to a gray curtain moving across the field of vision. Prior to the retina detaching, there may be black dots or squiggles floating across your vision, flashing lights and shadows or blind spots in the field of vision
Excellent information on retinal detachments is available from the American Academy of Opthalmology website here