The back of the eye is normally filled with a jelly called the vitreous. As the eye ages, the vitreous becomes less like a gel and more like a fluid. This is considered a normal effect of aging. Not all the jelly becomes fluid like at once, so small clumps or strands of the jelly may first form. When the eye moves, the fluid/gel moves as well and can eventually pull away from the retina and the optic nerve in the back of the eye. These clumps of jelly move around in the eye. As the light comes into your eye, it is blocked by these clumps of jelly, thereby casting a shadow onto your retina. An analogy would be how a shadow is cast on the ground as a cloud passes over the sun. When this happens in the eye, you end up seeing these shadows as floaters. Although the floaters appear to us to be outside of the eye, they are actually within the eye. The floaters can have many different shapes. While most often they appear as dots or lines, sometimes they can also appear like circles, clouds, cobwebs or even like a spider or a fly. Most of the time these objects will appear to move.
Sometimes, as the jelly pulls away from the retina, the tugging on the retina may also cause you to see flashing lights. In rare instances, as the jelly pulls away, it can rip a blood vessel or even rip a tear in your retina. Therefore, if you notice new flashes or floaters, an exam should be performed just to make sure that there are no other serious problems. Your doctor will perform a dilated examination and likely push on your eye (scleral depression) to make sure that no small holes or tears have developed in the retina.
No treatment is necessary for a PVD unless it leads to a retinal tear or detachment. There is no cure for floaters unless they are disabling or prevent normal day to day functioning, and only then would removal of the gel within the eye be recommended.