Ninjutsu

is one of the three main jutsu  categories. Ninjutsu is the most nebulous of the three, and may most simply be described as anything that is not genjutsu or taijutsu. Most ninjutsu require chakra and hand seals, but this is not always the case since the mere usage of weaponry qualifies as ninjutsu. The huge range of ninjutsu's uses is illustrated by its several sub-classifications: medical ninjutsu, reincarnation ninjutsu, space–time ninjutsu, barrier ninjutsu, sealing jutsu, cursed seal jutsu and sensory ninjutsu.

Most ninjutsu use shape transformation, which deals with controlling the form and movement of the ninjutsu, while nature transformation deals with changing the ninjutsu's physical properties. Ninjutsu that use nature transformation tend to follow a naming convention of their nature type followed by a more specific name. With practice and experience, certain ninjutsu can become second nature to a ninja, allowing them to perform it at will.

Background
Ninjutsu is humans' corrupted use of the Sage of Six Paths' ninshū originally created by his son, Indra; rather than use chakra to achieve understanding and create peace as the Sage intended, humans weaponised their chakra for war. Over time, shinobi came to believe that Hagoromo was the inventor of the ninjutsu rather than his eldest son.

Trivia

 * The simultaneous use of ninjutsu and taijutsu is called "nintaijutsu".
 * Jin no Sho describes Sage Art: Super Tailed Beast Rasenshuriken as "the most marvellous ninjutsu of all time".
 * Amado deems ninjutsu a mere attempt to replicate the effects of the Ōtsutsuki Clan's shinjutsu.