Shinobi as seen in the series.
Shinobi (忍) otherwise referred to as ninja (忍者) in the English translation of Naruto, are the primary focus and the main military power in the series. A female ninja is known as a kunoichi (くノ一). Most come from a hidden village, of which some come from specialized ninja clans, and will perform missions for a fee. These missions can include: doing manual labour, acting as escorts, gathering secret information, retrieving stolen items, carrying out assassinations, etc. In times of war, ninja will come together to defend their villages and land. The clearest characteristic of a shinobi is their ability to manipulate chakra to create and use jutsu.
Jutsu
Main article: Jutsu Jutsu (術, literally meaning: techniques/arts/skills) are the mystical arts that a ninja will utilise in battle. To use a jutsu, the ninja will need to use their chakra. To perform one, the ninja will bring out and release the two energies of chakra. By forming hand seals, the ninja is able to manifest the desired technique. Because of the extensive number of hand seals and different combinations, there are thousands of potential jutsu to be discovered.
Shinobi can use a variety of techniques often based on the utilization of chakra. This allows shinobi to perform extraordinary feats such as walking on water or walking up trees. Jutsu are separated into three groups, ninjutsu, genjutsu, and taijutsu. Ninjutsu implies perform a supernatural or beyond normal ability, the most common of which is the elemental jutsu. These are created by the user transforming their chakra into a specific nature or 'release'. The chakra types are Lightning, Earth, Fire, Wind, and Water. Other sub-types of chakra natures such as Wood Release and Ice Release also exist, created by combining two of the basic five releases. Taijutsu implies physical body techniques, which may or may not require chakra. Genjutsu manipulates the chakra in an enemy's mind and disrupts their chakra flow. Their senses get fooled which creates illusions, things that the foe believes to be real that are in fact fake. Genjutsu is the least-used of the three groups, possibly due to the difficulty required in performing the technique.
Tools
Main article: Ninja Tools
Ninja tools are used as alternatives to jutsu. They can include weapons, accessories, and even clothing. Ninja such as Tenten have mastered the use of such tools. A standard shinobi will have a pouch equipped with many such tools, and nearly every ninja is equipped with at least a shuriken holster. Most also possess and wear a forehead protector with the symbol of the village they derive from, though the ninja does not necessarily have to wear it over their forehead. The Inuzuka clan's ninken and Aburame clan's insects are also classified as tools.