What Are 'floaters' Appearing In Front Of One's Eyes?

Can you tell me what a "floater" is? I am referring to those little black things that can sometimes appear in front of one's eye. They are about the size of a fruit-fly. Strangely, both my husband and I got them at the same time this week. Mine went away after a few days. He tells me his has grown a tail! Just curious.

YYYY@YYYY .