Zanpakuto Facts

A Zanpakuto (literally soul-cutter sword) is the main weapon of the Shinigami, the Arrancar and the Vizard. The art of wielding a Zanpakuto is called Zanjutsu (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 Zanpakuto 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 Zanpakuto'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 (literally shallow hit) is 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 Zanpakuto, the "shallow hit", which refers to the grossly-reduced power of a Zanpakuto whose wielder cannot connect with it properly.

Zanpakuto Facts:

• Because they are part of their owner's soul, a Zanpakuto cannot be replaced, though it will slowly regenerate if it is broken.
• The Zanpakuto size reflects the amount of the swordsman's spiritual power.
• If a Shinigami pierces the chest of a human with their Zanpakuto and channels their Reiryoku through it into the Human's body, it will transform that human into a Shinigami temporarily. The act does not have a high chance of viability even when the human in question has a high level of spiritual energy, and if the method does fail the human will die.
• Every Zanpakuto has its own name. 
• A Zanpakuto can determine whether or not its wielder is worthy of its power.
• Normally, even a broken Zanpakuto would return to its original state as its wielder recovered.
• Zanpakuto have a true form. This true form resides inside their Shinigami. A Zanpakuto has a mind, and a body that controls their powers. Shinigami are able to grow more powerful by communicating with them. 
• Zanpakuto possess their own reiatsu, which can be sensed as any reiatsu can and is identical to that of their wielder though with subtle differences.
• Though rare, it is not impossible for more than one soul to be attuned to the same spirit. In such cases, ancient law of the Soul Society will not allow two beings to wield the same Zanpakuto, in which case, they must duel for ownership of the spirit, with the winner gaining it and the loser being sentenced to death.
• No matter what form (or size) a Zanpakuto takes, it is always virtually effortless for its owner to wield because it is a part of its owner's soul. Shinigami captains all consciously keep their Zanpakuto in a manageable size, otherwise they would be holding Zanpakuto the size of skyscrapers. Therefore, one cannot judge one's opponent's strength solely by the size of their Zanpakuto.

Sourced from http://bleach.wikia.com/wiki/Zanpakut%C5%8D