They are floaters: A blurry spot that seems to drift in front of the eyes but does not block vision.
The blur is the result of debris from the vitreous of the eye casting a shadow on the retina. The spot is the image formed by a deposit of protein drifting about in the vitreous, the clear jelly-like substance that fills the middle of the eye.
Floaters are often described by patients as spots, strands, or little flies. Floaters are usually benign (not serious). They can, however, occasionally result from a separation of the vitreous gel from the retina.
This condition is called a posterior vitreous detachment (PVD). Although a PVD occurs commonly, there are no retinal tears associated with the condition most of the time. No treatments are available for floaters although they tend to become less annoying over time.
Remember also that floaters are usually not associated with serious eye problems.
Permanent or recurring white or black spots in the same area of your field of vision may, however, be an early warning sign of cataracts or another serious eye problem.
If you experience a shadow or curtain that affects any part of your vision, this can indicate that a retinal tear has occurred and has progressed to a detached retina.
In this situation, you should immediately consult an ophthalmologist since time is of the essence.