Most of the time, age-related changes in your vitreous are the cause of eye floaters. As your eyes get older, the vitreous jelly deteriorates: it becomes more liquid, starts to sag and pull away from the inside of your eyeball, and some of the former gel clumps. The shadows of these stringy bits are what you see.

Floaters can also occur when the vitreous detaches from the surface of your retina. The stimulation of the retina during this process will often cause flashes in the eye. The moment the vitreous pulls away from the head of your optic nerve, it can make a ring-shaped floater appear temporarily.

Occasionally, this detachment will pull a bit of your retina with it. This retinal detachment leaks blood into your vitreous, which appears as a scatter of small dots and needs immediate attention from your ophthalmologist.

Bleeding and inflammation in the eye, from retinal tears, blood vessel problems or other injuries, tends to cause floaters in general. Floaters can also be small specks of protein and other material that was trapped in your eye as it was forming before birth.

Source: Coopervision