Cultured Freshwater Pearls Whats The Difference Between Pearl,cultured Pearl,freshwater Pearl And Freshwater Cultured Pearl?

Whats the difference between pearl,cultured pearl,freshwater pearl and freshwater cultured pearl? - cultured freshwater pearls

In view of a pearl ring, etc. Now for a whole, but confused by all the variants, is obviously wrong, imitation pearls are the other really?