Bleach is a long-running shonen series with a well-defined combat system, ranging from Zanpakuto to kido spells and Hollow powers. Zanpakuto are arguably the core of this combat system, forming the crux of a Soul Reaper's toolkit with their soul-cutting powers. But these are no ordinary weapons -- a Zanpakuto is a living weapon, more like a Soul Reaper's partner than a tool.
Zanpakuto have been a part of Bleach since the beginning, but Ichigo Kurosaki and the viewers alike understood very little about them until later arcs, like the Soul Society arc, began. The full story of Zanpakuto is a rather complex one.
The use of Zanpakuto, or soul-cutting swords, dates back well over a thousand years, when a man named Oetsu Nimaiya of Squad Zero began forging them with his unique skills. These Zanpakuto are made of hammered-out souls that begin as blank templates, or Asauchi. These generic, nameless Asauchi are given to Soul Reapers who join the Gotei 13 upon academy graduation, and from that point on, it's up to the wielder to unlock the blade's true potential.
A Zanpakuto, being a sword-shaped soul, is no ordinary weapon. It can think for itself and will form a partnership with its wielder if they make the effort to understand their weapon. To do so, a Soul Reaper can perform a meditation ritual with their blade, entering an inner world where they can learn their Zanpakuto spirit's name and personality, creating a lasting bond that boosts the cutting power and durability of the blade. Only a strong bond of trust and cooperation will allow a Zanpakuto to grow stronger, meaning that, if the Soul Reaper and their blade are out of sync or don't understand one another, they'll both be held back, spelling doom in battle. Kenpachi Zaraki, for example, obtained an Asauchi from a fallen Soul Reaper and didn't learn its name, leading to discord between the two and a cap on Kenpachi's combat prowess. Only when Kenpachi learned his blade's name, Nozarashi, did its true power emerge.
Ordinary Zanpakuto blades can be used as conventional katana, making proper sword technique paramount. In this form, Zanpakuto are mainly used against Hollows, along with kido spells. Once a Hollow is slain with a Zanpakuto, its spirit is purified and sent to Soul Society, though evil souls are sent to Hell instead. A Soul Reaper can also use their Zanpakuto to perform konso, allowing a regular soul, or Plus, to move on to the Soul Society. Ideally, a Soul Reaper will only need to perform konso during a patrol in the world of the living.
When a Soul Reaper understands their Zanpakuto well enough, its power will be drawn into Shikai mode. Shikai typically involves a Zanpakuto empowering its Reaper with a classical element, like fire, ice or wind. Other Shikai assume a melee-ready shape, as with Renji Abarai's Zabimaru. Shikai generally increases the Zanpakuto's attack power and unlocks new abilities, bring a Soul Reaper to all-new heights of power.
A select few Soul Reapers are born with the potential for Bankai as well, the Zanpakuto's ultimate form. Bankai mode takes years to unlock via strenuous training, and even more time is needed to fully master it. Bankai mode is five to ten times more powerful than shikai mode, but this comes with several drawbacks and risks. Using Bankai mode is highly taxing on the user's spirit energy, and if a Bankai is badly damaged or broken, it can never be restored to its original shape and function.
Notably, a Zanpakuto's Bankai will always have a thematic connection to its Shikai. For example, Toshiro's Shikai and Bankai are ice-based, and Byakuya's Bankai is a scaled-up version of its Shikai. Curiously, Captain Soi Fon's Shikai is a deadly stinger on her right middle finger, while her Bankai is more like a bazooka. It could be said that Soi Fon's bankai, Jakuho Raikoben, is an assassin tool in the sense that it deals all of its damage to a single target to annihilate them in one blow. It's simply not stealthy or quiet like its Shikai mode is.