Natural pearls come from freshwater or marine mollusks. They are called natural pearls because humans have no part in their growing process. Natural pearls are very rare today, with the majority of existing natural pearl jewelry being vintage.
Cultured pearls are also produced by freshwater or marine mollusks. They are just as real as a natural pearl, but instead they are farmed in process that is facilitated by humans. Because they are more reliable to produce, most jewelers today work with cultured pearls.