Visual floaters can be described as bubbles, strings, bundles of filaments which float within the visual field of one eye.
They are caused by small, benign inclusions in the vitreous.
They increase with age and may be permanent once present.
The sudden appearance of numerous floaters is a whole different ball game and may suggest retinal detachment.
Source
Tasman, W. et al. Duane's Ophthalmology. Lippincott Williams and Wilkins. 2008.