Naruto Uzumaki using a form of Ninjutsu, the Rasengan.
Ninjutsu (Literally meaning: Ninja Techniques) 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, and Cursed Seal jutsu.
Most ninjutsu use shape transformation, nature transformation, or both: shape transformation deals with controlling the form and movement of 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, "Fire Release: Great Fireball Technique" for example. With practice and experience, certain ninjutsu can become second nature to a ninja, allowing them to perform it at will.