Rarely are prokaryotic cells found by themselves. They often form communities composed of other microbes. These "biofilms" are collections of all these microbes in one place stuck together by the sticky slime they secrete (polysaccharide matrix), found attached to any kind of surface.
A few biofilms you may come into contact with often: the plaque on your teeth, the slime on river stones, the spots on contact lenses, etc.
More than 99% of bacteria live in biofilms, some of which are beneficial to humans, while others create some major problems. Humans have almost no method of getting rid of or destroying these biofilms.