Real pearls:
1. cultured pearls (i.e. farmed pearls)-- today's pearl market is a cultured pearl market.
2. natural (i.e. wild pearls, which form without any human intervention)-- while there are still some natural pearls, these are much more rare.
Real pearls form inside the mussel or oyster. The nacre is laid down by the animal itself. Some have an implanted shell nucleus and others do not.
Imitation pearls (a.k.a. fake, faux, simulated, etc.)
glass, plastic or shell beads coated with imitation pearl coating
The imitation pearl coating is most commonly made from an emulsion made from fish scale extract and a kind of lacquer. They are made in a factory.