What is the difference? There is so much misinformation about whether a cultured pearl is really a pearl. 

A pearl is formed when an irritant, such as a parasite or piece of shell, becomes accidentally lodged in an oyster's soft inner body, causing it to secrete a crystalline substance called nacre as a defense mechanism, which builds up around the irritant in layers until a pearl is formed. Whether a pearl is natural or cultured, they are both formed the same way.

The difference is that a cultured pearl is formed with some human intervention. That human intervention is the surgical process of implanting a nucleus (a shell bead that won't be rejected by the bivalve mollusk) on which the soft tissue muscle starts to coat with concentric layers of nacre. This is a protective defense that produces the beautiful pearl.