Soul Reapers are Shinigami that use their Zanpakutou's to cleanse the souls of beings that have been angered or have stayed in our world too long.
Each Zanpakutou is unique to it's wielder, each has its own name, and special ability.
Each sword has a released form (shinkai) and a Bankai, but only those Soul Reapers that have trained for many decades, sometimes centuries have the ability to call their Zanpakutou's bankai.