Floaters
Spots, or floaters, are small particles that float around in the vitreous (the jelly inside the eye), and the spot that you see is the shadow of these particles. They will move around when you move your eyes, and are usually more noticeable on light backgrounds. Most floaters, once there, will be there for a long time, however, if you see any new floaters, or flashing lights off to the side, you should have your eyes checked right away as these can be a sign of a retinal detachment.