The word "Keshi" means "poppy seed" in Japanese. The first keshi obtained came from the small Akoya oyster (Pinctada imbricata), thus the keshi pearls were also very small, like the little seeds of the poppy flower. This is now a common name used to identify a pearl that is produced by the oyster trough the use of the grafting technique, but in this case, the oyster got rid of the nucleus, by expelling it, but the mantle tissue survived and eventually formed a baroque shaped pearl with no nucleus inside of it. The nucleus acts as a "shape guider" for the pearl sac, and with nothing to "guide its actions" the oyster does as best as it can: usually baroque shaped pearls.
The main difference between a Keshi and a (nucleated) cultured pearl is the absence of a nucleus (shell-bead). The difference between a cultured and a natural pearl is basically the same (absence or presence of a bead). So, are a keshis and natural pearls the same thing? No. A keshi is a cultured pearl. Within its core you will find a sizable quantity of decaying organic matter (remains from the oyster's gonad), whereas a natural pearl has the decaying body of an intruder (and usually it is much smaller). Also, keshis came to be due to Human intervention. Keshis come in different size, shapes and coloration, and are specially sought after by those that seek a refreshing new look in their jewelry designs.