As the eyes move, the floaters move too. They do not follow your eye movements precisely, as they usually drift when the eyes start or stop moving. Floaters are caused by clumps or bits of material suspended in the vitreous jelly that fills the back of the eye. The floaters cast shadows on the light sensitive retina. It is actually the shadow of the floater that you see.
Flashes are sensations of light, when no light is really there.
Flashes may occur when the vitreous jelly pulls on or tears the retina. These flashes usually last for only a second or so, but typically occur repeatedly. They may be more obvious with eye movement, or in a dark room. Some people with migraine headaches may experience a different type of flash. These are usually shimmering, jagged lights that are present constantly for a period of fifteen minutes or so.