Fuuinjutsu or Sealing Arts
Fuuinjutsu or Sealing Arts, are a type of Jutsu that seals chakra, living beings, objects, energy, etc whithin another object through the use of kanji codes/spells.
Fuuinjutsu may vary greatly but most obey the same basic principle: You need a medium (living or not) in which was prepared the Kanji Codes/Inscriptions of the seal. You need to expend chakra to activate the seal and chakra to sustain it for a given amount of time. To release it, you must once again expend chakra to unlock the seal, while still needing the correct "key". The "key" is a variation of the technique only known to those who create it, thus disabling the probability of the seal being released by someone else; it can be a given set of handseals or even a DNA imprint (through a blood contract). However, even if most seals may work in this simple fashion, some more advanced seals work in many other ways. Even if you take into account that all seals must be triggered/fueled by chakra, they must all have a medium in which to seal away the target.
Because seals are fueled by chakra, they become weakened through time, as the chakra that was expended to create and maintain it, becomes consumed. Because of this, when sealing something in a living being, the seal is generally made to use the chakra from the being to sustain itself, delaying the eventual weakening.
Numerous users have been shown to possess like Orochimaru, Hiruzen, Jiraiya, Minato just to name a few. Another aspect of Fuuinjutsu which I really enjoy is barrier's, this being an aspect I would like to add to my shield some day. A lot of seals are focused on being activated by touch but there is so much more that could be achieved with this field of training and customs.