The zampakuto in bleach is the weapons of soul reapers, It is basically their souls taking the form if a sword. Each soul reaper posses a different zampakuto, and its size depends in the spiritual pressure (power) of the soul reaper.
The zampakuto is a living being inside the soul reaper it may take different form, as a human or even as something else like and animal or combination of animals. Each zampakuto has a name and for a soul reaper to fully wield it he must call out its name, this move is called its sword release form. In this form the sword changes form and can do all sort if things each peculiar to its own ability. Some ability can double the weight of an object, another can disintegrate into a million blade and so on...
