What is a Zanpakuto?
A Zanpakuto (斬魄刀; literally, soul-cutter sword) is the main weapon of the Shinigami, the Arrancar and the Visored. The art of wielding a Zanpakuto is called Zanjutsu (斬術; literally, swordsmanship).

Zanpakuto are the trademark weapons of the Shinigami. Capable of cutting spiritual bodies, they are amongst the few weapons which can be used to combat Hollows. Each Shinigami carries a Zanpakuto, and each Zanpakutō is unique: as the swords are both reflections of a Shinigami's soul and power, and sentient beings unto themselves. The
Zanpakuto's name is also the name of the living spirit that empowers the sword and lends its strength to the Shinigami who wields it. These beings can vary greatly in appearance and have their own distinct personalities which match their owner's. Normally, the Zanpakuto spirit can only be seen by its wielder.  Otherwise, they dwell in their own "inner world", created within the minds of the Shinigami who wield them. As such, each Shinigami's inner world is drastically different from another Shinigami's and is unique to themselves. A Zanpakutō's spirit can bring its wielder into its inner world, though Shinigami can voluntarily achieve this simply by meditation.

A Zanpakuto's shape and abilities are based on their Shinigami's soul. Once a Shinigami learns his/her sword's name, they are able to communicate with one another and grow stronger together. Zanpakuto are born with their Shinigami and they die along with their Shinigami. Shinigami use them in battle as a reflection of their heart. When released, they can display a vibrant power. They are one with the Shinigami, and they share a Shinigami's conviction. Asauchi (浅打ち; literally, "shallow hit"): The class of nameless Zanpakuto that all low-class Shinigami wield. Shinigami who are still in training - those who do not yet belong to any of the Gotei 13 - all wield generic Zanpakuto. Shinigami who are unable to communicate with their Zanpakutō, the "shallow hit", which refers to the grossly-reduced power of a Zanpakutō whose wielder cannot connect with it properly.