Naruto Uzumaki's Jinchūriki Forms

When Naruto Uzumaki taps into the Nine-Tailed Demon Fox's chakra, it increases every physical aspect of his body. Increased healing, near limitless chakra, an almost impenetrable shield, and chakra so dark, it's poisonous. Naruto has unique access to the demon fox's chakra due to the seal the Fourth Hokage placed on him, which sealed the fox's Yin chakra, giving Naruto control over the Yang chakra.

Jinchūriki Form
By simply tapping into the demon fox's chakra, Naruto's blue eyes turn red, his nails and incisors grow long and sharp, and his whisker marks become more feral. Depending on how much chakra is unleashed, red chakra could glow around his body in an untamed way. Naruto first reached this form, when Sasuke Uchiha was apparently killed by Haku causing pure rage to take over allowing the demon fox's chakra to leak out of the seal.

Even though this is the weakest state of his Jinchūriki forms, it is still a deadly foe as it increases his physical strength well above most shinobi (one example was being able to easily shatter Haku's ice mirrors when Sasuke's fireballs had no effect). Until special training from Jiraiya, Naruto could only reach this state through rage or if his life was in imminent danger but after having a inner talk with the demon fox, he is able to tap into the demon fox's chakra on his own, or by simply telling the fox to give him some power.

One-Tailed Transformation
During Naruto's battle with Sasuke at the Valley of the End, when Naruto was at the mercy of Sasuke, the demon fox told Naruto he should be grateful to him and gave him more chakra. The amount of chakra was so great that it it leaks out of his body in a denser state. All the physical traits of the basic Jinchūriki form are present with one major bonus; the red chakra becomes a complete shield around his body, in the shape of a fox, complete with long ears and a tail. In his one-tailed transformation, Naruto fights more like a beast. Instead of using his fists, he instead uses his sharpened claws. While he can stand on his own two feet, he can move at greater speeds on all fours. But his most powerful weapon is the chakra itself. The bubbling chakra appears to have a mind of its own as a strike that was meant for Sasuke missed, the chakra itself stretched off Naruto's arm and hit Sasuke before he could react. As the chakra as a mind of its own, it makes it extremely difficult, if impossible to predict Naruto's movements, as Sasuke was able to see Naruto somewhat perfectly, but couldn't predict what the chakra would do with the Sharingan. The chakra can stretch far from the body, as Naruto was shown using his "arms" to attack Sasuke from great distances, at one point being on an opposite side of the valley, and still attacking him with the chakra arms. The shield around his body appears to be impenetrable, comparable to Gaara's Ultimate Absolute Defense: Shield of Shukaku.

Two-Tailed Transformation
After catching the Akatsuki member, Deidara, and beating what he thought was Deidara to pieces only to realize he had been attacking a substitution, his anger at losing Deidara and not being able to avenge Gaara's death caused him to slip into his two-tailed transformtion. His physical characteristics match the first two forms, but his red eyes and incisors grow larger, his lips get a black outline, similar to fox gums. The chakra shield's only difference between the one-tailed transformation and the two-tailed is the extra chakra tail that has grown. While in this state, Naruto is still in control of his body, though to what extent is unknown. In the anime, he attempted to attack Kakashi Hatake, when the later tried to calm him down, and possibly in his mind, tried to stop him from finding Deidara. The amount of chakra being produced by the two-tailed transformation is so great, that even Naruto's shadow clones, doubled over in pain from the chakra.

Three-Tailed Transformation
After Orochimaru revealed himself to Team Yamato, and after baiting Naruto with news of Sasuke, Naruto went on a rage and first attacked him in his one-tailed state. When that failed and due to further baiting from Orochimaru, Naruto skipped the two-tailed transformation all together, and went straight into his three-tail transformation. The three-tailed looks exactly like the two-tailed, again the only difference being the number of tails. In this stage, Naruto's conciouss becomes very difficult to reach, as it begins to become surpressed by the Kyubi's. The cellular damage of this state becomes more intense than the first two tails. Naruto began doubling over as the chakra felt like his insides were burning. Desdpite this, the form does possess great power. It allows for two new abilities, as well as upgrades on the orginal abilities. The first is a Genjutsu that makes the victim seem like they're being tortured by the Kyubi, and the second is the ability to combine roars with chakra, creating supersonic waves to blast the opponent.

Four-Tailed Transformation
When Orochimaru faced the three tails, it proved no match for him. In desperation and anger, Naruto turned to the fox's power and started to enter its grasp.

When Naruto entered this state, his conciouss nearly completely dissapeared, as it only returned after Naruto injured Sakura. The cellular damage of this state is intense, as Naruto's skin begins burning off, his blood mixes with the Kyuuibi's chakra, giving it a dark appearance, as Naruto's skin is replaced with a layer of Kyuubi chakra. Naruto's eyes also transform into ghostly white circles. This makes Naruto merely a skeleton for the Kyuubi's chakra.

In this state, Naruto gets intense power. In addition to an upgrade on his other abilities, Naruto's claws become poisonous due to Naruto's blood being mixed in the cloak. Despite this, the poison can only cause paralysis, and it takes time for it to affect its victim. Naruto also gains the ability to shoot bubbles of chakra from his body, and fire beams of chakra from his mouth. So far, the only ninja capable of easily defeating and potentially killing Naruto in this form is Orochimaru (though he left Naruto alive since he could help defeat Akatsuki), as Jiraiya was only evenly matched with it. Only the Mokuton abilities of Yamato were able to surpress the chakra when Naruto was in this form, and he could only do so because of the necklace of the First Hokage.

In the end, despite being the strongest Jinchūriki form seen so far, the four-tailed transformation is also Naruto's greatest downfall. After the form recedes, the chakra begins healing Naruto rapidly. Jiraiya assumes that if that continues to happen to Naruto, his body will break down and his life span will shorten.

Jinchūriki Chakra Control
In the event of the demon fox manifesting, Naruto's sensei, Jiraiya, prescribes a special written seal that is to be placed on Naruto's body. This seal suppresses the demon fox's chakra and restores Naruto to his normal self. In addition, Yamato has demonstrated a technique that will reverse Naruto's demon fox transformation, enhanced greatly by the First Hokage's necklace that Naruto wears.

Sasuke also demonstrates the ability to forcefully suppress the demon fox's chakra. To parallel the fox's apparent connection with the Uchihas, Itachi Uchiha of Akatsuki is originally assigned to capture it as part of the organization's goal of obtaining all the tailed beasts, but this task is later given to Pein.